US investigators have told how two suspected pressure-cooker bombs sprayed nails and metal pellets into the crowd at the Boston Marathon, killing three people and injuring more than 175.
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Investigators combing through the grim aftermath of the terrorist attack have also found evidence that timing devices were used to detonate the bombs that ripped through race spectators on Boylston Street, an official briefed on the investigation said.
ABOVE: How the Boston bombings were captured on camera
Working with fragments painstakingly gathered from Boston's largest-ever crime scene, investigators determined that the two bombs were probably fashioned from six-litre pressure cookers, filled with nails and small ball bearings to increase the carnage, and then hidden in black nylon bags or backpacks which were left on the ground.
Seventy people remain in Boston hospitals after the attack, including 24 people in critical condition.
Pictures have emerged of what appears to be a bag on a Boston footpath shortly before one of two blasts shattered the marathon on Monday.
The FBI, which is leading the investigation into the bombing, asked the public for tips on anyone who may have been lugging a heavy black bag near the finish line. Local and federal law enforcement officials are scouring thousands of photographs and video for some sign of the bomber.
"The person who did this was someone’s friend, co-worker or neighbour," said Richard DesLauriers, the special agent in charge of the FBI’s Boston office.
"Somebody knows who did this." No one has claimed responsibility for the atrocity and "the range of suspects and motives remain wide open," Agent DesLauriers said.
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The FBI confirmed nylon fragments found at the site of the bombing may be from bags used to conceal the deadly devices.
The Boston Globe reports a circuit board, which may have been used to detonate the bombs, has also been recovered by investigators. The low-tech bomb design has been promoted by al-Qaeda to its radicalised American followers.
Boston television station WHDH-TV released photos taken soon before the explosion showing a bag lying next to a mailbox and against a race-route barricade. A second image, blurred by WHDH-TV because of its graphic nature, suggested the explosion was centered where the bag had been placed. The viewer who took the picture, according to WHDH, said there may have been a lag of an hour or more between the two images.
Agent DesLauriers more than 2000 tips from the public had been received since the terrorist bombing. Evidence recovered from the scene was undergoing laboratory tests, he said.
White smoke from the blasts indicated that the bomber used so-called smokeless or black-powder explosives rather than a military-style high-explosive such as C-4, which produces a distinctive black smoke, according to Fred Burton, former deputy chief of counterterrorism for the State Department’s Diplomatic Security Service who investigated the first World Trade Centre bombing.
Doctors tell of injury horror
The bombs, while crude, were designed to cause maximum carnage, with some blast victims left with more than 40 pieces of shrapnel embedded in their bodies.
The fragments were uniform, indicating that they came from the two bomb blasts and not from the surrounding environment, said George Velmahos, chief of trauma surgery at Massachusetts General Hospital.
A team of 20 Massachusetts General surgeons, including one who had himself just completed the 42.2km race, amputated four legs above the knee. Across Boston, at least 11 people underwent amputations after the attack.
VIDEO: Eight year-old Martin Richard is the first victim to be named in the Boston Marathon blast, as his sister and mother still fight for life in hospital.
‘‘We just completed what the bomb had done,’’ Dr Velmahos said of the amputations.
''If these victims had spent even a few more minutes at the scene they would not be alive today.
"There are a variety of sharp objects that we found in their bodies. Probably these bombs had multiple metallic fragments in them and we removed pellets and nails."
The devices were "placed probably low on the ground, and therefore lower extremity injuries are to be expected.''
Authorities in Boston continued scouring what they say is the most complex crime scene in the history of the Massachusetts capital, as details emerged of the victims who died and were severely wounded.
Martin Richard, an 8-year-old from nearby Dorchester, was among the dead in the blast that also injured his mother and sister.
A second victim, Krystle Campbell from Arlington, was named by the Boston Globe overnight.
In a terrible twist, Ms Campbell's parents had believed for hours after the blasts that she was injured but alive, only to find out doctors had confused the 29-year-old with her best friend.
Meanwhile, the Chinese Consulate in New York confirmed a Chinese woman was the third person killed in the blasts. She was studying statistics at Boston University. A second Chinese student is in a serious condition in hospital.
The official Chinese news agency Xinhua reported that relatives have requested that the deceased not be identified.
Culprit may be 'homegrown terrorist'
Police admitted they had no suspects, and no organisation claimed responsibility but President Barack Obama said authorities would find those responsible.
''This was a heinous and cowardly act and, given of what we now know took place, the FBI is investigating this as an act of terrorism,'' the president said in a statement in the White House briefing room. ''We don't have a sense of motive yet, so everything at this point is speculation.''
While investigators don’t know the origin of the attack, initial evidence suggested a homegrown terrorist may be behind it, Georgia Senator Saxby Chambliss, the top Republican on the Senate Intelligence Committee told reporters after a national intelligence briefing.
‘‘There are a lot of things that are surrounding this that would give an indication it may have been a domestic terrorist,’’ Mr Chambliss said. However, he said officials couldn't say for certain whether the attack originated in the US or was the result of a plot overseas.
Agent DesLauriers, who is in charge of the city's FBI office, said officials would be working for days at the scene of the attack.
''We will go to the ends of the earth to identify the suspect or subjects responsible for this despicable crime,'' he said.
Investigators asked residents of Boston to hand over any photos and video of the scene of a bombing that killed three people at the Boston Marathon.
''We encourage the continued submission of any photographic evidence,'' DesLauriers said.
Boston Police Commissioner Ed Davis said security footage had been taken from nearby businesses.
''Even as we were removing victims, officers were assigned to go into the local establishements and secure those videos,'' he said.
A day after the attack the FBI and Boston police declined to reveal details of their probe, or whether they suspected it was linked to foreign or domestic extremists.
A team of about 30 bomb specialists and sniffer dogs combed the site, the finish line for the famous marathon in the north-east US city, as officials appealed for witnesses to call in with any relevant information.
"The scene is going to take several days to process," said Gene Marquez, acting special agent in charge of the Bureau of Alcohol, Tobacco, Firearms and Explosives office in Boston.
Police said the area had been swept twice before the race and no explosives were found.
At a press conference about 26 hours after the attack, Boston mayor Thomas Menino called on local residents to rally together.
‘‘We will not let terror take us over,’’ he said.
