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

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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’.

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.

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

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

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

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.'

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

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

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

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

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

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