Nails and other shrapnel appear to have been packed into the bombs that killed three people and injured more than 170 others in Boston, doctors have said.
Several people have had to have limbs amputated and others are at risk of losing legs following the blasts that ripped through crowds during the city's marathon.
Doctors at Massachusetts General Hospital, one of several hospitals where victims were taken, told reporters eight people were in a "severe" condition.
A total of 17 people remain in a critical condition.
Meanwhile, officials have confirmed there were no unexploded devices found, despite earlier reports of multiple bombs in the area.
Nails and other shrapnel have been removed from patients' bodies
The FBI said it was following a number of leads following the attacks but special agent Richard DesLauriers declined to say whether anyone was in custody.
"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," he said.
Mr DesLauriers was speaking at a news conference in which Boston mayor Thomas Menino pledged the city would recover and said: "Yesterday, terror was brought to the city of Boston."
Speaking at a news conference, Barack Obama said the twin-bombing was an "act of terror".
Martin Richard, eight, was among three people killed
He said: "What we don't yet know, however, is who carried out this attack or why, whether it was planned and executed by a terrorist organisation, foreign or domestic, or was the act of a malevolent individual."
He added: "What I have indicated to you is what we now know. We know it was bombs that were set off. We know that obviously they did some severe damage. We do not know who did them.
"We don't have a sense of motive yet. So everything else at this point is speculation."
Mr Obama said the American people "refused to be terrorised" and that stories of "kindness" in the aftermath of the blasts had proved that.
He urged people to come forward if they knew anything that might help the authorities to bring to justice those responsible for what he called a "heinous and cowardly" act.
Bystanders rushed to the aid of victims
Among those killed in the blasts in the heart of the city was an eight-year-old boy, Martin Richard, who lived in Boston.
The Boston Globe newspaper reported that the boy had been waiting at the finish line for his father, who was running the race.
The bombs went off within seconds of each other and about 100m (330ft) apart on the same street, blowing out windows and sending smoke and debris into the air.
Emergency workers ripped down fencing and carried away seriously injured men and women amid scenes of panic and confusion.
Harrowing stories have emerged of victims whose limbs were torn off by blast forces.
Liz Norden, a mother of five, told the Boston Globe how two of her sons had each lost a leg in the blast.
Both had gone to Boylston Street to see a friend finish the race.
"Ma I'm hurt real bad," Ms Norden quoted one of her sons as telling her in a phone call from the ambulance.
The explosions happened four hours into the race and about two hours after the men's winner had crossed the line, as amateur runners were reaching the finish.
The FBI says it is following a number of leads
More than 17,000 competitors had completed the race by the time the blasts struck.
Police commissioner Ed Davis said the area around the blast areas is the most complex crime scene in history of the department.
Authorities are now looking for amateur video and photographs that can give clues to who set off the bombs. One Boston home has been searched as part of the investigation.
Security has been stepped up in Washington and New York, and Boston itself is a city on high alert, with armoured vehicles seen on the streets on Tuesday and random checks of backpacks on public transport.
Earlier, a plane was grounded at the city's Logan Airport over security concerns.
An injured man on the streets (Pic: Kenshin Okubo/Daily Free Press)
There have been no immediate claims of responsibility for the attack, the most serious in the US since the 9/11 World Trade Centre atrocity. Al Qaeda-linked groups and militant white extremists have attacked targets in America in the past.
The Pakistani Taliban, who have previously threatened attacks in the US, have denied any involvement.
More than 25,000 people were registered as taking part in the race, 374 of whom were British. There were also 108 Irish athletes.
One British runner, Abi Griffiths, 34, from London, told Sky News the scene was in a "state of chaos".
She said: "Police were everywhere, we were being evacuated out of the area and it was really eerie."
The British Foreign Office has said it is not aware of any British nationals who have been injured but that it will continue to monitor the situation.
Police in London are now reviewing security plans for this Sunday's London Marathon - the next major international marathon.
The London race's chief executive, Nick Bitel, said it was "a very sad day for athletics and for our friends in marathon running".
Julian Assange gives a speech from a balcony of the Ecuadorean embassy 
Tremors were felt in Karachi, Pakistan, where office workers were evacuated 
High-rise buildings in Dubai were evacuated after the earthquake 
More than a hundred people were injured in the blast 
Unclaimed runners' bags tell a tragic tale 
Alpe d'Huez is a popular ski resort 
