Investigators are searching for the person or group responsible for planting two bombs at the Boston Marathon finish line that killed three people and wounded more than 100 others.
Police and Federal Bureau of Investigation (FBI) officials searched a house in connection with the bombings late on Monday. Massachusetts state police have confirmed that a warrant was served to search the property in the suburb of Revere in Boston.
Law enforcement agencies are hunting for clues, while critically wounded victims of Monday's blasts fight for their lives and others contemplating a new life as amputees.
The bombs ripped through the crowd at the finish line of the Boston Marathon, shattering the annual civic celebration on the Patriots' Day state holiday
, while tens of thousands of people packed the street to cheer on the runners.
The two devices using gunpowder as explosives were packed with ball bearings and other shrapnel to maximise injuries, a senior law enforcement official, who declined to be identified because of the sensitivity of the case, briefed on the investigation.
No one has been arrested in regard to the bombing. A White House official said the case would be treated as "an act of terrorism", but it had yet to be determined whether the attack came from a foreign or domestic source.
President Barack Obama promised to catch whoever was responsible.
"We still don't know who did this or why," Obama said, adding, "Make no mistake: We will get to the bottom of this."
"It is a criminal investigation that is a potential terrorist investigation," said Richard DesLauriers, the FBI special agent in charge for Boston.
As many as two unexploded bombs were also found near the end of course as part of what appeared to be a well-co-ordinated attack, but they were safely disarmed, according to a senior US intelligence official, who also spoke on condition of anonymity because of the continuing investigation.
Many runners were heading for the finish when a fireball and smoke rose from behind cheering spectators and a row of flags representing the countries of participants, video from the scene showed.
The two explosions came moments apart four hours and nine minutes into the race, long after the world-class runners had finished but around the time average contestants were arriving.
'Legs blown off'
"I saw people who looked like they had their legs blown off. There was a lot of blood over their legs. Then people were being pushed in wheelchairs," said Joe Anderson, 33, a fisherman from Pembroke, Massachusetts, who had just run the race holding a large US flag.
Many of the victims were gravely injured, Massachusetts Governor Deval Patrick said.
| A witness describes the aftermath
Some suffered shrapnel wounds and amputations and will require repeat operations in the coming days, said Peter Fagenholz, a trauma surgeon at Massachusetts General Hospital.
Doctors treated 29 patients including eight who were in critical condition, Fagenholz said.
Many of the seriously injured patients had suffered bone, soft tissue and vascular damage to their legs, he said.
"We're seeing a lot of shrapnel injuries" from small metal debris, Fagenholz told reporters outside the hospital.
There had been "several" amputations, he said.
The dead included an eight-year-old boy, the Boston Globe reported, citing two law enforcement sources briefed on the
A two-year-old with a head injury was being treated at Boston Children's Hospital, the hospital said in a statement.
The blasts put police on alert in major cities across the US, including in Washington, DC, and New York City, sites of the September 11 attacks.
The annual Boston Marathon, held since 1897, attracts an estimated half-million spectators and about 20,000 participants every year.
'Really, really bloody'
Several competitors and race organisers were crying as they fled the chaos.
"There are a lot of people down," said one man, whose running number 17528 identified him as Frank Deruyter.
Runners who had not finished the race were diverted straight down Commonwealth Avenue and into a family meeting area, according to an emergency plan that had been in place. The race was later abandoned.
Al Jazeera's Josh Rushing, reporting from downtown Boston, said that there were a number of people there with "just a shocked look on their face".
"I think everyone is trying to put the media-pieces back together, and try to figure out how they are going to move forward," he said.
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