An investigation is under way after two explosions near the finishing line of the Boston Marathon left three people dead and at least 140 injured.
The FBI has taken over co-ordination of what it described as a “potential terrorist inquiry”.
Boston police say officers are working around the clock and all leave has been cancelled.
At least 17 people are critically wounded, officials say, and the injuries include several amputations.
One of the dead was an eight-year-old boy, US media said.
In a televised address, President Barack Obama vowed to bring those behind the attack to justice.
“We will find out who did this. We’ll find out why they did this,” he said.
“Any responsible individuals, any responsible groups, will feel the full weight of justice.”
Although he did not use the word “terrorism”, a White House official later said: “Any event with multiple explosive devices – as this appears to be – is clearly an act of terror, and will be approached as an act of terror.”
The first explosion came at about 14:50 local time (18:50 GMT) on the north side of Boylston Street, about two hours after the winners crossed the line.
There was initial confusion and panic. Some runners fell to the floor while police and bystanders ran to help those caught in the blast.
Then seconds later, another explosion ripped into the crowd further away from the finishing line, between 50 and 100 metres (yards) from the first blast.
TV footage showed bloodied runners and spectators being treated at the scene and the road strewn with debris. Rescuers tore down temporary fencing to reach the casualties.
Medical officials speak of at least 140 injured, with injuries including severed limbs, shrapnel wounds, cuts and bruises,
At an initial news conference, Boston Police Commissioner Edward Davis said authorities had received “no specific intelligence that anything was going to happen”.
He urged people to stay indoors and not congregate in large groups.
At a second news conference he said the death toll now stood at three. He said that no suspects were in custody.
A search thought to be related to the inquiry was under way at a flat in a Boston suburb, Boston media reported.
Police said a search warrant was issued on Monday in the Revere district but provided no further details.
The White House and security experts are being cautious as to whether domestic terrorist groups or jihadists were behind the attacks, says the BBC’s security correspondent, Gordon Correra, in Washington.
The device itself is thought to be fairly crude and there is a very clear focus on finding the evidence rather than speculating, he adds.
Massachusetts Governor Deval Patrick said Boston would be “open” on Tuesday but that there would be “a heightened law enforcement presence”.
“There will be random checks of backpacks and other parcels. We are also asking that everyone be on a state of heightened vigilance,” he said.
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