FBI gives its agents images of TWO Boston bombing 'suspects': Hunt for men seen wearing back-packs near the marathon finish line before terror attack

FBI gives its agents images of TWO Boston bombing 'suspects': Hunt for men seen wearing back-packs near the marathon finish line before terror attack - Investigators have reportedly identified a Boston marathon suspect using security video and cellphone records - Much of key video footage came from Lord and Taylor department store - near the site of Monday's second blast - Video shows man in black with shredded pants running away in the aftermath of one of the blasts - Shows the suspect walking towards the mail box, dropping a package and walking away - New crime scene photographs from the first blast confirms that a pressure cooker was used in the device - Lid of pressure cooker found on rooftop of building 35 yards away - Other photographs submitted to the FBI reveal the scene before and after the second bomb detonated - Devices were designed to act as 'Claymore' anti-personnel devices - which are meant to maim on the battlefield - The pressure-cooker bombs were packed with shards of metal, nails and ball bearings - Eight-year-old boy, 29-year-old woman and University of Boston grad student killed in the attack - 183 people injured, at least 23 of them in critical condition and 'a lot' of amputations have been performed

A suspect in the Boston bombings has been caught on CCTV apparently planting the second of the two explosives in what investigators have hailed as a major breakthrough in the case.

Video obtained by the FBI shows the suspect placing a bag near the Lord and Taylor department store while talking on a cellphone moments before the twin blasts ripped though the city centre, killing three and injuring more than 180.

The unidentified man is also captured using his phone at the moment the first explosion detonated 100 yards away on Boylston Street on Monday and 12 seconds before the second blast close to where he is standing.

He was wearing a black jacket, a grey hooded sweatshirt, is about 6ft or 6ft 2ins tall and of a medium build. He had a white or off-color-white baseball cap on backwards.

The footage led the FBI to announce it had made 'substantial progress' in the investigation and are now trawling phone to track the suspect down.

They stressed no-one has been named or arrested, but are reluctant to make the video public in case it forces the man to go underground.

In another key development, pictures have been released of a second potential suspect who is seen fleeing the site of one of the blasts as everyone around him cowers in terror clutching their ears.

Investigators search for evidence on the rooftop of a building located above the site of a bomb blast on Boylston Street 

This picture was taken from exactly the same angle as the camera from Lord and Taylor which is believed to have shown someone walking towards the mail box before dropping a package and walking away 

Dressed all in black, the man's pants have been shredded and he hurriedly looks from left to right before making off in the opposite direction to the where members of the public are aiding the injured.

An FBI circular was sent out to law enforcement agencies last night seeking help to identify the two men.

According to CBS, investigators are now going through cell phone logs to determine who made calls from the location near the Lord and Taylor store the time of the explosions. 

Other sources have said the FBI is working with a list of names of cell phone owners and attempting to match one of those to the unknown man on the surveillance. 

Authorities have said they are seeking this man either as a witness or indeed as a possible suspect and do not know his name. At this point, federal officials stressed no arrests have been made.

And late on Wednesday evening, CNN and Fox News reported that they had seen images designated for 'law enforcement eyes only' that show two men they consider to be of 'high interest'.

According to a reporter who has viewed the images, one of the men is wearing a backpack that matches one of the recovered bags believed to be have been used in the terror attack - while the other individual also is seen to have a bag.

Second Bomb: Photos show a bag next to a mailbox along the marathon route and may have been the footage used by the FBI to positively ID a suspect

Shocking: Seconds after the bombs went off on Boylston Street there is no sign of the bag, the picture is blurred because of the graphic nature of the content

Is this the man? While everyone seems to instinctively cower - the man dressed in black and highlighted - appears to run away at speed from the scene in the opposite direction to everyone else who was caught up in the blast 

Could this be the bombing suspect? The man runs away as everyone around him instinctively falls to the ground or covers their ears

No arrest has been made but the FBI believe they can see the suspect drop the bag and run away from video footage 

As those caught up in the blast appear to help the stricken victims of the bombing - the man in black and tattered clothes can be seen sprinting away from the scene 

City Council President Stephen Murphy, who said he was briefed by Boston police, said investigators saw the image on surveillance footage they got from a department store near the finish line and matched the findings with witness descriptions of someone leaving the scene.

'I know it's very active and very fluid right now — that they are on the chase,' Murphy said. He added: 'They may be on the verge of arresting someone, and that's good.'

The bombs were crudely fashioned from ordinary kitchen pressure cookers packed with explosives, nails and ball bearings, investigators and others close to the case said. Investigators suspect the devices were then hidden in black duffel bags and left on the ground.

As a result, they were looking for images of someone lugging a dark, heavy bag.

One department store video 'has confirmed that a suspect is seen dropping a bag near the point of the second explosion and heading off,' Murphy said.

A law enforcement official who was not authorized to discuss the case publicly and spoke to The Associated Press on the condition of anonymity confirmed only that investigators had an image of a potential suspect whose name was not known to them and who had not been questioned.

Evidence: Authorities viewed video footage from the Lord and Taylor department store

The pressure cooker lid was found on top of the Charlesmark Hotel which is 35 yards away from the explosion site - such was the force of the blast

Map showing the Boston crime scene perimeter where experts are trying to piece together the clues left behind the deadly marathon explosion 

And according to CBS News senior correspondent John Miller the big debate right now among law enforcement is whether to release the image of the man to the public.

'It's been a tough call,' said Miller. 'For investigators, there's always a difficult choice because if the person doesn't know you're looking for him, he may stay in place, you may catch up to him,' said Miller.

'If he does know you're looking for him, he may run. On the other hand, if you don't get him, it's always great to enlist 20 million or so more eyes in the public who may be able to give you a location right away. 

'That's not going to happen tonight anymore. It was going to happen earlier -- they've rethought that. It may happen with the release of that picture tomorrow. They're reassessing.'

In a turbulent day in the US, Washington was plunged into a major security alert after a letter addressed to Barack Obama was found to contain the deadly poison ricin.

A bomb squad scoured the US Capitol, parts of which were placed on lockdown, after two more suspicious letters were found in Senate office buildings, one addressed to Republican senator Richard Shelby. 

A ricin-laced letter was sent to another senator, Roger Wicker, on Tuesday which – like the one to the President – had a Tennessee postmark and was signed ‘I am KC and I approve this message’.

Has the Boston bomber's cellphone given him away? How the FBI uses high-tech triangulation to track criminals

Cell phone triangulation is a well-known tracking method within the wireless industry and is utilized by law enforcement authorities across the nation 

Michael Barker, who works with Cell-Loc - a mobile tracking firm explained how individuals can be tracked by their mobile phone devices 

'Every time the cell phone is on, it periodically sends a little registration message to the phone company, `Here I am! Here I am!' Barker said. 

'That message contains the cell phone's electronic serial number and tells the service provider when the phone has drifted in and out of cell tower range. 

Federal agents then easily can get in contact with the cell phone service company and get the location of the nearest cell tower in contact with the activated phone 

Law enforcement then can equip agents with devices designed to triangulate the signal and determine its location within about a third of a mile and the direction it was traveling in, Barker said. 

There were reports that an ‘emotionally disturbed’ man with letters in his backpack was held and questioned at a Senate building.

Last night, a suspect from Mississippi had been arrested in connection with the letter.

The FBI said it had found no link between the letters and the Boston bombing, but its refusal to rule out a connection plunged a rattled America into further uncertainty over national security. 

The US Secret Service stressed that the ricin letter got nowhere near the President and was intercepted at a mail sorting centre in Maryland.

But tensions are already high across the US after Monday’s bombing and are likely to reach fever pitch when Mr Obama visits Boston today for an inter-faith memorial service for those who died.

The twin explosions on Monday killed three people and injured 183 – America’s worst domestic terror attack since the destruction of the World Trade Centre in 2001.

A press conference due to be held at 5 p.m. by Boston police commissioner Ed Davis was postponed - no reason was given for this decision, but officials with said they hoped to re-schedule it later on Wednesday night.

And by late afternoon there were conflicting reports about the color of the man in question - CNN suggested initially that he may be 'dark skinned', while CBS News claimed he is white.

Because of that video - and local news footage - authorities were able to get exactly what they have been looking for - an individual coming in and dropping a package in a black bag before quickly walking away again. 

It was initially believed the man had been arrested and would soon be taken to court but White House officials later denied that anyone had been take into custody or even positively identified.

CNN - who broke the arrest news - later backtracked and said it had been a misunderstanding and no arrest had been made.

However, the Associated Press stood by their reports that someone was taken into custody, even after it was widely disputed. 

According to the LA Times, two suspects have been singled out in surveillance videos of the scene. A federal official said he has been briefed that authorities believe a second video or photo showed 'two men with two backpacks'.

Though CNN's John King said sources told him it was a 'dark-skinned man', CBS reported that it was a white man who was wearing a black jacket, a gray hooded sweatshirt and a white or off-white baseball cap backwards. 

He is said to be six feet or 6-feet-2-inches tall with a medium build. However, federal officials vehemently denied that anyone had been taken into custody.

'Despite reports to the contrary there has not been an arrest in the Marathon attack,' Boston police said from an official Twitter account.

The FBI added: 'Since these stories often have unintended consequences, we ask the media, particularly at this early stage of the investigation, to exercise caution and attempt to verify information through appropriate official channels before reporting.'

Soon after the false arrest stories from CNN were reported, a bomb threat was called into the federal courthouse and the building - as well as the surrounding area - was evacuated.

Bomb: The photos were produced by the Joint Terrorism Task Force of Boston and provided to Reuters by a U.S. government official who declined to be identified and show pieces of a stainless steel pressure cooker with an Underwriters Laboratory number. One picture shows the imprint: Gas and Electric

The devices are believed to be pressure cooker bombs — a commonplace cooking utensil in many countries — a piece of which is shown on the left along with remnants of the black nylon bag and on the right, pieces of the sophisticated detonator 

This picture obtained by Fox 5 Atlanta is believed to be the black back-pack that is thought to have held one of the explosive devices which detonated at the finishing line of the Boston Marathon on Monday

The photographs appeared to confirm that the trigger mechanism of the Boston marathon bombs were indeed battery powered - with experts claiming they showed a high level of sophistication 

This picture reportedly shows a number of ball bearings which were packed into the pressure cooker to cause maximum maiming potential 

Destroyed electronic board parts lie on the ground around the scene of the detonation of the first Boston marathon bomb - in photographs obtained anonymously by Reuters and produced by the FBI

Fagor Duo Stainless Steel Pressure Cooker which is believed to have been the type used in the bombing, left, and the graphic right, shows how the household appliance would have been used to cause maximum damage

Amateur pictures taken by someone in a nearby office building may show the man responsible running away from the scene - though this is only speculation.

'I went to the window and I was looking in the direction of the finish line. I saw simultaneously a runner go down, a huge explosion, and then a deafening roar,' Benjamin Thorndike said. 'I had my camera in my hand, and I just pushed the rapid-shutter button down and just took 25 pictures over the course of what felt like a long time, but I think it was only 15 or 20 seconds.'

He added that he thought it was strange that the man was running away from the scene while everyone else was instinctively crouched down and taking cover.

It also emerged today that the force of the first blast at the marathon was so strong, the lid of the pressure cooker bomb was found on the sixth-floor roof of a hotel 35 yards away from the explosion site and is now a vital clue in the investigation. 

A guest at the Charlesmark Hotel discovered the crucial piece of evidence just minutes after the blast. He picked up the twisted metal – believing it was a hubcap from a vehicle damaged in the bomb – and gave it to a policeman.

Twenty-four hours later he was quizzed by FBI agents, who revealed the mangled metal was one of biggest clues so far in the search for the terrorists who killed three and injured 183 others.

Terror: Visitors flee the U.S. Senate's Hart office building after two suspicious packages were found on the U.S. Capitol

Evacuated: Washington was plunged into a major security alert after a letter addressed to Barack Obama was found to contain the deadly poison ricin 

Hotel owner Mark Hagopian said: 'One of the guests had been up on the roof earlier in the day. Immediately after the blast he went back up there and spotted what he thought was a hub cap.

'As it hadn’t been there earlier he thought it could have come from a damaged car, so he picked it up and took it downstairs.

'He handed it to a policeman and thought nothing more of it until the FBI contacted him and hotel manager Curt Butcher on Tuesday evening.

'They met for 35 minutes and the FBI confirmed that it was part of the pressure cooker bomb.'

Hogopian, 50, was hosting a marathon party for 100 people when the first bomb exploded on Monday afternoon.

As guests rushed outside the see what was happening, the second bomb went off just yards from the hotel’s outdoor patio. 

'We were knocked off our feet by the blast and all around us there were bodies – seven or eight people missing limbs’ he said. 'There was blood splayed everywhere. It was utter chaos.

'One of the fourth-floor guests went up to the roof to see if he could see what was happening. That’s when he found what he thought was the hubcap.

'He handed it to the cop as everyone was being evacuated – at the time it was feared there were more bombs set to go off.

'Now it turns out that piece of metal is a big piece of the evidence. Apparently, forensics should be able to get many more clues from that. The FBI also took away video and photos the guest had taken. They are scouring those for further evidence.'

Lockdown: A bomb squad scours the US Capitol after two more suspicious letters were found in Senate office buildings, one addressed to senator Richard Shelby

Held: There were reports that an 'emotionally disturbed' man with letters in his backpack was held and questioned at a Senate building

New crime scene photographs of the remnants of the first bomb which detonated during Monday's Boston marathon shows that a six-liter pressure cooker was used in at least one of the deadly charges - as experts described the devices as military-style 'anti-personnel' devices.

The images - released by the Joint Terrorist Task Force - show the wreckage of a stainless steel pressure cooker with an Underwriters Laboratory safety mark and an imprint that reads gas or electric, with experts describing the devices as military-style 'anti-personnel' devices.

Furthermore, it is being claimed the deadly devices used were designed to act like 'homemade claymores' - powerful, directional anti-personnel devices.

Described as using a 'low explosive', most likely black or smokeless powder, the bombs are reported to have exploded outwards with shrapnel at 3,300 feet per second. The lid of a pressure cooker was found on a nearby rooftop and investigators were able to pinpoint which type of cooker was used.

One brand of pressure cooker with '6L' on the bottom is made by the Spanish company Fagor, which sells about 50,000 of the six-liter pots in the United States every year, according to the New York Times.

This gives investigators a real chance to narrow down their list of suspects - which they said yesterday was 'wide open' - and find out who is responsible for the worst attacks on U.S. soil since 9/11.

Investigators are still combing through the area for more vital clues to try and find the person or persons responsible for Monday's bomb 

Investigation: Officials survey the site of a bomb blast on Boylston Street in Boston searching for evidence that might help them find a suspect or suspects 

They will use every clue, from the cooker's manufacturer and retailers to the types of nails used in the shrapnel, to try and find out from where the bomb parts were purchased and by whom.

Roy Parker, a retired Bureau of Alcohol, Tobacco, Firearms, and Explosives agent who ­developed the agency’s explosives training program, said examiners are looking at scraps of the bomb components, bags and all other forensic evidence.

He said: 'You’re looking for a needle in a haystack, but the needle is there. If you look long enough, you’ll get stuck with it. This is not an unsolvable crime.'

The bombs had the power to propel metal shards great distances, designed primarily to maim - like a military 'claymore' device.

The resulting explosions sent metal tearing through skin and muscle, destroying the lower limbs of some victims who had only shreds of tissue holding parts of their legs together when they arrived at the emergency room of Massachusetts General Hospital, doctors there said.

Initial examination of the bombs revealed they appeared to utilize battery packs and circuit boards, which indicates a sophisticated triggering mechanism with an 'egg-like' timer.

'It appeared to be built from scratch but with a sophisticated triggering mechanism. And frankly, at the end of the day, all bombs are crude devices, and it is the way they are triggered that can be sophisticated,' said one official with strong knowledge of explosives. 'They functioned as designed.' 

Forensic investigators look through the top of a building on Boylston Street for clues, two days after the bombs exploded just before the Boston Marathon finish line

Low-quality explosive ingredients may have been the bombs’ key components, according to an unnamed expert quoted by CNN. That would explain the yellowness of the flame, as shown in video and photos of the event.

Higher-quality military explosive is usually gray or black, said the expert.

Photographs taken by a member of the public of the second bomb site before and after detonation show an orange and grey bag next to the barrier - right where the bomb appears to have gone off.

Detectives are now investigating the possibility that this bag contained the device that exploded with such devastating effect. 

However images and fabrics from the remains of the bomb shows they were kept in black nylon duffel bags so it is as yet unclear if the bomb was in he gray and orange bag.

Vigils were held overnight as hundreds of people gathered to remember the victims, the third of whom was named yesterday as Chinese Boston University student Lü Lingzi.

Lingzi was attending the Boston marathon with her friend Zhou Danling, a student of actuarial science at BU, who was originally said to be in a coma at Boston Medical Center but showing signs of improvement after suffering serious injuries in the blast.

The other two fatalities in the bombing have been identified as eight-year-old Martin Richard, the son of a Dorchester community activist, and 29-year-old restaurant manager Krystle Campbell. 

A five-year-old boy is still critically injured at Boston medical center.

A law enforcement official told CBS News that the two bombs that exploded were made to look like discarded property. 

Earlier in the day, lead FBI agent Richard DesLauriers confirmed that items from the first blast have been recovered and are being sent to a special facility at FBI headquarters in Quantico for study - in the hope that they will lead them to the killer or killers.

He also admitted the investigation into those responsible was in its 'infancy' and that the range of suspects remained 'wide open'.

As those caught up in the blast appear to help the stricken victims of the bombing - the man in black can be seen to make his way at speed away from the scene 

And as the circumstantial evidence builds up around the devastating attacks builds, one counterterrorism expert told said that the Boston bombings bore all the hallmarks of the 2004 Madrid attacks - albeit on a lesser scale.

'This has the hallmarks of a Madrid-style event in many respects,' Rossini told The Daily Beast. 'We don’t know who the bombers are, but I assume it was more than one. It’s just my gut that there might have been two people involved.'

The Madrid commuter train bombings were a coordinated attack that used multiple explosive devices which killed 191 people and wounded 1,800 on March 11th, 2004.

They were attributed to an al Qaeda affiliated terror cell.

'I’m not sure if it’s domestic, or if there was shrapnel or ball bearings in the bombs,' Rossini said, 

'But when I saw a still photo of the explosion, there was what looked like a fireball from an accelerant like a propane tank or gasoline”—yet another similarity to the carnage in Spain. 

'Of course, a pipe bomb could have that effect as well.'

Saying that the bomber or bombers most likely dropped the bags easily amongst the commotion during the race, Rossini said that the FBI will be looking to the remnants of the bags to lead them to the killers.

Rossini, however, cautioned against jumping to any conclusions that the bombing was the result of Islamic-inspired terrorism. 'It could be a white supremacist group. We just don’t know.'

The Shenyang Evening News reported Wednesday on its official Twitter-like microblog account that the victim is named Lü Lingzi. An editor at the newspaper says that Lü's father confirmed his daughter's death

People gather for a vigil for the victims on the Boston Common as an investigation continues into dual bombings at the Boston Marathon finish line in Boston, Massachusetts, on Tuesday

More than 2,000 tips from the public were reviewed and analyzed in the first 24 hours but no individual or group has yet claimed responsibility for the attack. The FBI has issued a plea to members of the public for help in the hunt for clues saying: 'Someone knows who did this'.

He said among those items were 'pieces of black nylon which could be from a backpack and fragments of BBs (ball bearings) and nails possibly contained in a pressure cooker device. 

'The bag would have been heavy because of the components believed to be in it. We won't know with certainty until the laboratory completes its final review.'

The second device was also housed in a metal container says the document, but so far there is not enough evidence to confirm for certain if a pressure cooker was also used.

'Someone knows who did this,' added Richard DesLauriers, the special agent in charge of the FBI’s Boston office. 

'Importantly, the person who did this is someone’s friend, neighbor, coworker or relative. We are asking anyone who may have heard someone speak about the marathon, or the date of April 15, in any way that indicated that he or she may have targeted this event to call us.'

DesLauriers added: 'At this time, there have been no claims of responsibility.'

Earlier he vowed to go 'to the ends of the earth' to find those responsible.

In video footage also handed over to the FBI, an unidentified man is seen to flee the scene of one of the blasts as everyone else cowers in the seconds after the explosions.

Dressed all in black, the man's pants have been shredded and he hurriedly looks from left to right before making off in the opposite direction to the where members of the public are aiding the injured.

In a press conference held today, Boston Police Commissioner Ed Davis confirmed that the marathon route had been swept twice by bomb detection units and declared clear. 

'[But] people can come and go and bring items in and out after the races started,' said Davis.

People stand during an interfaith prayer service remembering the victims of Boston Marathon bombings at the Paulist Center on Tuesday evening in Boston, Massachusetts 

Disaster: Mourners attend candlelight vigil for Martin Richard at Garvey Park, near Richard's home in the Dorchester section of Boston, on Tuesday, April 16

Vigils were held overnight as hundreds of people gathered to remember the victims, the third of whom was named yesterday as Chinese Boston University student Lü Lingzi

The two bombs that killed three people and injured at least 183 at the Boston Marathon on Monday were made from six-liter pressure cookers crammed with shards of metal, nails and ball bearings and stashed in black backpacks, police sources said today.

The cruelly-designed bombs have 'frequently' been used in Afghanistan, India, Nepal and Pakistan, according to a 2010 Homeland Security Department pamphlet - hinting at the origins of the bombers behind the worst terrorist atrocity in the U.S. since 9/11.

A history of pressure cooker bombs and IEDS since the 1990s: How the crude device popular with al-Qaeda and the Taliban has traveled to the United States

First reports of pressure cookers being used as bombs date back to the 1990s when Maoist rebels used them during the Nepalese civil war that began in 1996. 

During the early part of the last decade, pressure cooker devices were being used in the creation of IEDs across Afghanistan and Pakistan - and were used with alarming frequency against coalition forces in Afghanistan. 

However, because pressure cookers are not as common in the United States as other developing nations, the Department of Homeland Security issued a 2010 warning to report seeing any in public places unattended. 

The 2010 warning was released several months after the failed May 2010 Times Square bombing - which used one as part of its mechanism. 

In 2011 U.S. Army Private Naser Jason Abdo was charged in plotting to set off a pressure cooker bomb on Fort Hood, Texas 

He reportedly learned how to construct the device from reading al Qaeda's online magazine, Inspire 

When the devices exploded near the crowded Boston Marathon finish line around 2.50pm on Monday, victims suffered as many as 40 shrapnel wounds each and at least 10 people needed amputations. Witnesses described seeing body parts flying through the air and shoes that 'still had flesh in them'.

And today, FBI investigators said that they will attempt to reconstruct the deadly bombs that brought havoc to the streets of Boston.

During the day, Federal law enforcement sources have briefed that the two devices were indeed made from pressure cookers and hidden in back-packs.

The recovered pieces suggest that the two bombs were identical and a Boston law enforcement source said that parts of a circuit board were also found - which could have been the method to detonate the devices.

As the evidence strands are brought together, experts have said that despite pressure cooker bombs being used as IEDs by the Taliban and al Qaeda in Afghanistan, it doesn't mean that other people can not discover how to create them.

So far, no one has claimed responsibility and DesLauriers said, 'The range of suspects and motives remains wide open.'

In Washington, Senator Saxby Chambliss, the ranking Republican on the Senate Intelligence Committee, told CNN that the facts point to a domestic, home-grown source.

'There are a lot of things that are surrounding this that would give an indication that it may have been a domestic terrorist, but that just can't be assumed,' Chambliss said.

On Tuesday afternoon, a second victim killed in the blasts was identified as 29-year-old Krystle Campbell, from Arlington, Massachusetts. It comes after another was named as eight-year-old Martin Richard. The third victim has not yet been named - but is believed to be a Chinese graduate student of Boston University.

A Chinese student is confirmed to be the third person killed in the Boston Marathon Bombings. The Chinese Consulate, in New York, is not releasing the students' name at this time

Though the victim has not been named at the request of the woman’s family, she attended Boston University and had gone to watch the marathon with two friends.

The survivor, identified by the consulate as Zhou Danling, is in stable condition at Boston Medical Center.

'She cannot talk now but can communicate with pen and paper,' the consulate said in a statement.

'This is a painful and tragic lesson, but we will learn from this as well,' said Massachusetts Governor Deval Patrick on Tuesday..

Patrick also announced that President Barack Obama would attend an interfaith service honoring the victims of the tragedy at 11 a.m. on Thursday at Cathedral of the Holy Cross in South Boston. 

Collecting evidence: Two men in hazardous materials suits put numbers on the shattered glass and debris as they investigate the scene at the first bombing on Boylston Street

The bombs used to kill and maim are believed to have contained black powder or gunpowder as the explosive, and information on how to make such a bomb is available on the internet, experts said. The devices were then left at the scene to look like discarded property, CBS News reported.

Investigators have also found pieces of an electronic circuit board which could indicate a timer was used in the detonation.

Although no group has yet claimed responsibility for the attacks, similar devices were used in the failed 2010 attempt to bomb Times Square by Faisal Shahzad, who admitted he had undergone bomb-making training at a militant Islamist faction camp in Pakistan.

A pressure-cooker bomb is also a preferred weapon of al-Qaeda and listed as the 'most effective' weapon of jihad, according to an English-language terror magazine called Inspire, in an article entitled 'How to Build a Bomb in the Kitchen of Your Mom'.

Another article in Inspire last year listed 'the most important enemy targets' for jihadists in America - and included sporting events, CNSNews noted.

The aim should be to target 'human crowds in order to inflict maximum human losses,' a terrorist known as Abu Musab al-Suri wrote. 'This is very easy since there are numerous such targets such as crowded sports arenas, annual social events, large international exhibitions… etc.'

Killed: Eight-year-old Martin Richard from Dorchester, Massachusetts was among the three people killed when two bombs detonated at the Boston Marathon on Monday

Loss: Krystle Campbell, pictured, also lost her life in the terrorist attack. Doctors originally confused her identity with a friend, so her parents believed she was alive

If a pressure cooker was used, it probably cost around $100 to construct, say former federal forensic and explosive investigators. It's like a pipe bomb but bigger and more powerful.

Also on Tuesday, Homeland Security Secretary Janet Napolitano said there is no sign the explosions in Boston were part of a 'broader plot,' but her department is still being cautious.

'While there is no current indication to suggest that the events in Boston are indicative of a broader plot.

'Out of an abundance of caution,' she added, 'DHS continues to keep in place enhanced security measures at transportation hubs, utilizing measures both seen and unseen.' 

Martin Richard, 8, who was waiting at the finish line with his father, mother and siblings, was among those killed when the two bombs detonated at the crowded sports event. His mother has undergone brain surgery and his six-year-old sister lost a leg. His 12-year-old brother escaped injury.

Neighbors told MailOnline about the moment Martin Richard's father, Bill, returned home still wearing hospital scrubs on Monday night and looking like the 'walking dead' as he struggled to come to terms with his son's death and injuries suffered by his wife, Denise, and daughter, Jane.

In a statement released on Tuesday, he said: 'My dear son Martin has died from injuries sustained in the attack on Boston. My wife and daughter are both recovering from serious injuries.

Blast: Runners continue to run towards the finish line of the Boston Marathon as the first of two explosions erupts near the finish line of the race 

Exact Moment: People react as the second explosion goes off near the finish at the Boston Marathon finish line on Monday, sending authorities out on the course

'We thank our family and friends, those we know and those we have never met, for their thoughts and prayers. I ask that you continue to pray for my family as we remember Martin. We also ask for your patience and for privacy as we work to simultaneously grieve and recover. Thank you.' 

Doctors confused another victim, Krystle Campbell, 29, with her friend, so for hours her parents believed she was alive and undergoing surgery. Only when they went to visit her in hospital, did they realize the patient was not their daughter.

Loved: Peace is written on the sidewalk in front of the Richard house in the Dorchester neighborhood of Boston, left, while neighbors embrace in the street

Sympathy: Flowers and gifts are left by loved ones outside of the Richard family home in Dorchester, Massachusetts on Tuesday morning

First pictures: Martin was killed in the first explosion as he waited to give his father a hug at the finishing line. His sister and mother were also injured

Tragic: Martin Richard, right, is pictured with his family. His mother, Denise, has undergone brain surgery and his six-year-old sister Jane lost a leg in the blast, while his older brother, Henry, 12, escaped injury. They are also pictured with his father, Bill

A LOOK INSIDE THE HOSPITALS: HOW ARE THE VICTIMS?

Boston Children's Hospital
8 injuries; 0 critical condition
Ages from 2 to 15 years old
Massachusetts General Hospital
29 injuries; 8 critical condition; 4 amputations
Ages from 28 to 71
Brigham and Women's Hospital
31 injuries; 5 critical condition
Beth Israel Deaconess Medical Center
24 injuries; 4 critical condition; 13 in serious condition
Tufts Medical Center
9 injuries
Boston Medical Center
23 in total; 19 still remain at the hospital
7 critical condition; 6 serious; 6 fair
Please check back for updates 

'My daughter was the most lovable girl,' her father, William Campbell Jr. said. 'She helped everybody, and I'm just so shocked right now. We're just devastated. She was a wonderful, wonderful girl. Always willing to lend a hand.'

Another, unnamed victim was also killed in the blasts.

Twenty three of the injured remain in a critical condition and CNN reported that at least 10 victims have needed amputations. At least nine of the wounded are thought to be children, including a two-year-old boy who suffered a head injury.

Another child hurt in the blast was 11-year-old Aaron Hern, of Martinez, California, who was hit by flying shrapnel in his thigh as he waited for his mother to cross the finish line. He is being treated at Boston Children's Hospital and is expected to undergo further surgeries.

Doctors treating the wounded agreed that it appeared the devices had contained sharp metal.

Investigators do not know of a motive for the bombings, nor do they have a specific suspect or anyone in custody. In a desperate move, they have appealed to members of the public to submit photographs and cell phone footage of the marathon and explosions.

Earlier reports noted that officials had arrested a Saudi national, who is being treated for burns and shrapnel wounds, in connection to the bombings after a civilian saw him acting suspiciously at the scene.

But now officials have revealed the man, Abdulrahman Ali Alharbi, 22, is being regarded as a witness, rather than a suspect, the Washington Post reported.

It comes after 20 police and federal officials raided his home on Monday night, but the search turned up nothing.

After the twin detonations ripped through the cheering crowds, one witness told CNN that it 'felt like a huge cannon' and other described horrifying scenes of screaming spectators, missing limbs and unresponsive bodies.

Hunt: Police and federal officials exit an apartment complex at 364 Ocean Avenue with a possible connection to the earlier explosions that occurred during the Boston Marathon on Monday

Sorting through: Bags of people's belongings are sorted near to the Boston Marathon finish line on Tuesday as investigations continue into the bombings

WAS THE TIMING SIGNIFICANT? PATRIOTS' DAY IS REVERED BY RIGHT-WING GROUPS OPPOSED TO THE FEDERAL GOVERNMENT

• Yesterday's attack took place on Patriots' Day, which marks the first battle of the Revolutionary War and the 'shot heard 'round the world.' It is a day held in reverence by right-wing domestic groups and others who oppose the federal government. 
• The 1995 bombing of the Oklahoma City federal building, which killed 168, occurred the day after Patriots' Day. Bomber Timothy McVeigh was said believe the date was significant. 
• The federal siege of the Branch Dravidian compound, which left 82 members of the cult dead, began on Patriots' Day in 1993. 
• Today is also Tax Day, when federal income tax returns are due. In recent years, it has been seized on by members of the Tea Party as cause to protest federal government policies and tax rates. 
• A Justice Department source tells MailOnline that authorities are looking into the possibility that the Boston attack, which killed three and injured 183 more at the finish line of the Boston Marathon, could have been planned to coincide with Patriots' Day. 
• President Barack Obama himself acknowledged the significance of the holiday during his briefing this afternoon. 
• 'Today is Patriots Day,' he said. 'A day that reflects the freedom Boston has celebrated throughout its history.' 

'Bodies and body parts. Blood all over,' District Fire Chief Ron Harrington of the Boston Fire Department's District 3 said.

'A little boy lying in the street. A young woman in her twenties. Both dead. It was mayhem. I saw two people with arms hanging loose, and one without a leg. A shoe with flesh still in it.'

At a press conference outside Mass Gen hospital on Tuesday morning, Dr George Velmahos, chief of trauma surgery at the hospital, revealed that eight patients between the ages of 28 and 71 remained in critical condition at the hospital.

Among them four people had undergone amputations, and he said they were still trying to save two legs. Other injuries included wounds from 'pellets, shrapnel, nails', which he believed came from the bombs. Some victims were hit by '10, 30, 40' pieces and most injuries were to the lower extremities.

'The experience is obviously overwhelming,' he said. 'We are all extremely sad. We are suffering emotionally for what happened to the people of Boston and many others. But at the same time, we can't feel but proud because the medical community here in Mass Gen responded in an amazing way.'

He described one doctor who ran the marathon but then, despite being dehydrated, went straight to work, while others jumped on planes to help out.

Doctors at Brigham and Women's Hospital added that they had treated 31 patients from the bomb blasts and that five remain in critical condition.

Initially counter-terrorism sources in the city believed that seven devices were planted across the city - but only two detonated.

A law-enforcement official said late on Monday evening that investigators believe other packages were simply left behind as runners and pedestrians rushed away from the scene in the aftermath of the blasts.

A federal law enforcement official told CNN that both bombs which detonated at the Boston finish line were small, and initial tests showed no C-4 or other high-grade explosive was used - indicating they were crude devices.

In a press conference on Tuesday morning, Obama reiterated authorities are still unaware of who was behind the attack - whether it was the work of an organization or a 'malevolent individual'.

For the first time he acknowledged it was an act of terrorism.

'We will bring them to justice,' he vowed, recounting stories of kindness and heroism following the blasts.

'The American people refuse to be terrorized... If you want to know who we are, who America is, how we respond to evil, that's it: Selflessly, compassionately, not afraid.'

In another press conference on Tuesday, Boston officials said the investigation continues with a 'multitude of resources on the street'.

Special Agent in Charge Richard DesLauriers said there are no known additional threats, while Suffolk County District Attorney Daniel Conley branded the attack 'an act of cowardice'.

An aerial graphic that show how the tragic events of today unfolded at the finish line of the Boston Marathon 

Map of the bomb sites in relation to the city and the marathon which was run on Monday

Injured people and debris lie on the sidewalk near the Boston Marathon finish line following an explosion in Boston, Monday, April 15, 2013

Horrific Injuries: Medical workers aid an injured man at the 2013 Boston Marathon who screams out in pain 

'We will go to the ends of the Earth to identify the subject or subjects who are responsible for this despicable crime -- and we will do everything we can to bring them to justice,' DesLauriers said. 

Commending the efforts of volunteers and emergency responders, Mayor Menino added: 'I've been mayor for 20 years now and I've never seen... people pull together like this. It's a tragedy, but Boston is a resilient city.'

After the twin detonations ripped through the cheering crowds, witnesses described the horror.

The fiery twin blasts took place about 12 seconds and about 100 yards apart, knocking spectators and at least one runner off their feet, shattering windows and sending dense plumes of smoke rising over the street and through the fluttering national flags lining the route. 

Blood stained the pavement, and huge shards were missing from window panes as high as three stories.

An injured person is helped on the sidewalk near the Boston Marathon finish line following Monday's two bomb blasts 

The explosions ripped into an idyllic afternoon finish for the marathon. The first men had passed the finish line 2 hours and 10 minutes after the staggered start, and the first women crossed just 16 minutes later.

The first blast sent a quick plume of smoke two stories high. Runners nearby stopped in their tracks, confused and unsure. After a few seconds later, a second explosion happened a half-block away, with a deep boom caught on television cameras.

Emergency personnel rushed to the area, and the street was quickly sealed off.

'I saw it go off and smoke billowed up. Everyone just stopped and hunched down,' said Pam Ledtke, 51, from Indianapolis, who was about 75 yards from the finish line when the explosions went off. 'They didn’t know what to do. All of a sudden, people were screaming.'

One doctor, Allan Panter, stood near to the finish line said he was 25-feet away from the first blast when it detonated.

CELEBRATION TURNED TO TRAGEDY: HOW THE HORROR AT THE BOSTON MARATHON UNFOLDED

• 9 A.M. - The 2013 Boston Marathon begins.
• 11:58 A.M. - Rita Jeptoo of Kenya becomes the women's winner, crossing the finish line with a time of 2:26:25.
• 12:10 P.M. - Men's winner Lelisa Desisa, of Ethiopia, finishes with a time of 2:10:22.
• 2:50 P.M. - Two blasts go off only seconds apart at the finish line, less than 100 yards from each other on the north side of Boylston Street.
• 3 P.M. - A fire breaks out in the mechanical room at the JFK Presidential Library, but is reportedly not related to the explosions at the marathon.
• 3:34 P.M. - The Massachusetts Bay Transportation Authority announces multiple sections of the Green Line will be suspended due to police activity.
• 4:02 P.M. - The first official reports from the Boston Police Department list two dead and 23 injured.
• 4:30 P.M. - A third blast goes off near the JFK Library though no one is injured. Authorities say the explosion is being treated as an 'ongoing event.'
• 4:30 P.M. - Boston Police Commissioner Edward Davis holds a press conference urging people to remain indoors and not congregate in large groups.
• 5:23 P.M. - The fire in the JFK Library is put out.
• 6:10 P.M. - President Barack Obama addresses the nation, vowing: 'We'll find out who did this. We'll find out why they did this.'
• 6:30 P.M. - One of the dead is reported to be an 8-year-old boy.

Walking Wounded: A man staggers away from the scene of the explosions at the 2013 Boston Marathon in Boston

'I saw at least six to seven people down next to me,' he said. 'They protected me from the blast. One lady expired. One gentleman lost both his (lower) limbs. Most of the injuries were lower extremities.'

Nickilynn Estologa, a nursing student who was volunteering in a block-long medical tent designed to treat fatigued runners, said five to six victims immediately staggered inside. Several were children; one was in his 60s.

'Some were bleeding from the head, they had glass shards in their skin,’ she said.

'One person had the flesh gone from his leg; it was just hanging there.’ Another woman, she added, was lying on a gurney as emergency personnel raced through the tent, giving her CPR.

Horrific Scene: The aftermath of the Boston Marathon bombing on Monday evening in the city

'I just can’t believe anyone would do something like this,’ Estologa said.

Veteran marathon runner Bill Iffrig, 78, was almost at the finish when 'the shock waves just hit my whole body and my legs just started jittering around.'

Iffrig, can be seen wearing an orange tank top and falling to the ground in video of the explosion, and was helped to his feet by an event volunteer and had just a scratch from his fall, he told CNN.

Right after the blasts, police officers could be seen carrying bloody spectators to medical tents intended for exhausted runners in desperate attempts to save lives.

'They just started bringing people in with no limbs,' said runner Tim Davey of Richmond, Va. He said he and his wife, Lisa, tried to shield their children's eyes from the gruesome scene inside a medical tent that had been set up to care for fatigued runners, but 'they saw a lot.'

Speech: President Barack Obama spoke on Monday evening, saying that the American people stand by those in Boston affected by today's bombing

'They just kept filling up with more and more casualties,' Lisa Davey said. 'Most everybody was conscious. They were very dazed.'

Authorities in New York and Washington tightened security precautions in the wake of the blasts. 

The Bureau of Alcohol, Tobacco, Firearms and Explosives sent all of its bomb technicians, explosives officers, explosives specialists and canine officers from their Boston and New York field divisions to the scene, as well as some investigators from Washington.

The U.S. Federal Bureau of Investigation, Justice Department, Homeland Security Department and other agencies were all lending assistance to the investigation, authorities said.

Obama directed his administration to provide whatever assistance was necessary, the White House said.

Spectators typically line the 26.2 mile race course, with the heaviest crowds near the finish line.

Lucky Escape: Boston Marathon runner Bill Iffrig appears on Piers Morgan on CNN after the marathon explosions on April 15, 2013

Seventy-eight-year-old Bill Iffrig, who fell to the ground in the seconds after the blasts, has become one of the unfortunate icons of the Boston marathon bombings

The Boston Marathon has been held on Patriot's Day, the third Monday of April, since 1897. The event, which starts in Hopkinton, Massachusetts and ends in Boston's Copley Square, attracts an estimated half-million spectators and some 20,000 participants every year.

Of the 23,326 runners who started the race on Monday, 17,584 finished before the blast, marathon officials said. The runners were diverted before officials brought the marathon to a halt.

The Boston Marathon released a new statement on Tuesday afternoon, extending its sympathy to families affected by the bomb blasts and thanking all those who helped save lives.

'What was intended to be a day of joy and celebration quickly became a day in which running a marathon was of little importance,' it said.

'Boston is strong. Boston is resilient. Boston is our home. And Boston has made us enormously proud in the past 24 hours... We are committed to continuing that tradition with the running of the 118th Boston Marathon in 2014.'

A man is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square on April 15, 2013 

A person who was injured in an explosion near the finish line of the 117th Boston Marathon is taken away from the scene in a wheelchair

An unidentified Boston Marathon runner leaves the course crying near Copley Square

This article originally appeared in : FBI gives its agents images of TWO Boston bombing 'suspects': Hunt for men seen wearing back-packs near the marathon finish line before terror attack : dailymail.co.uk : By Rachel Quigley, Lydia Warren, Annette Witheridge In Boston, Massachusetts and James Nye : PUBLISHED: 16:43 GMT, 17 April 2013 | UPDATED: 12:13 GMT, 18 April 2013