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 their officers are working around the clock and all leave has been cancelled.
No group has said it carried out the attack, which left at least 17 people critically wounded, officials say.
One of the dead was an eight-year-old boy, Martin Richard. Several of those injured have had to have limbs amputated.
In a televised address, President Barack Obama vowed to bring those behind it 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, about 170m 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.
At least 140 people were injured, with injuries including severed limbs, shrapnel wounds, cuts and bruises.
At an initial news conference on Monday evening, Boston Police Commissioner Edward Davis said authorities had received "no specific intelligence that anything was going to happen".
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.
Details of the victims have also not been officially released, but eight-year-old Martin Richard, from Boston, has been named as one of those killed.
His mother and sister were also injured as they waited for his father to finish the race.
Officers are examining video and photographs from the marathon and have appealed for people to hand over their own videos and photos to help with the investigation.
Unconfirmed reports said two other unexploded devices were found near the end of the race but were safely defused.
State police officer Roupen Bastajian had just finished the race when he heard the blasts.
"I started running toward the blast and there were people all over the floor," he said.
"We started grabbing tourniquets and started tying legs. At least 25 to 30 people have at least one leg missing, or an ankle missing, or two legs missing."
A doctor at Massachusetts General Hospital said "several amputations" had been performed there.
As a massive security operation swung into operation, the Federal Aviation Administration barred low-flying aircraft from within 3.5 miles (5.6km) of the site.
Key sites in Washington DC and New York have also seen security tightened.
Shortly after the blasts, a fire broke out at Boston's John F Kennedy Library a few miles away from the explosions.
Police said the blaze might have been caused by an incendiary device but it did not appear to be related to the explosions.
The annual Boston Marathon this year had a field of about 23,000 runners and was watched by hundreds of thousands of spectators.
It is held on Patriots Day, a Massachusetts state holiday which commemorates the first battles of the American Revolution in 1775.
British police said they were reviewing security plans for Sunday's London Marathon - the next major international marathon - but that it was going ahead, saying they were "very, very well-practised" at managing big events.