An investigation is underway after two explosions near the finishing line of the Boston Marathon left three people dead and more than 100 injured.
The FBI has taken over co-ordination of what it described as a "potential terrorist inquiry" into the worst bomb attack on United States soil since September 11, 2001.
Investigators are examining the remains of the explosive devices but so far nobody has been detained. They are also poring over video and photographs from the race for clues.
The Marathon is run by 27,000 people and watched by half a million every year, so officials expect plenty of visual evidence into what happened before and after the attacks.
Officials say more than 140 people are wounded, at least 17 of them critically, and the injuries include several amputations.
The authorities say no motive has emerged nor has anyone claimed responsibility for the attacks.
In a televised address, the United States President, Barack Obama, has vowed to bring those behind the attack to justice. "We will get to the bottom of this and we will find out who did this. We will find out why they did this."
Vice President Joe Biden said: "Our prayers are with those people in Boston who have suffered injury."
Boston Police Commissioner Edward Davis said "no specific intelligence'' had been received that anything was going to happen. He urged people to stay indoors and not congregate in large groups.
At another news conference later in the day, he said the death toll now stood at three, including an eight-year-old boy. He said that no suspects were in custody.
Massachusetts Governor Deval Patrick confirmed reports that more than 100 people were injured, some gravely. He said Boston would be "open" on Tuesday but that there would be "a heightened law enforcement presence".
The FBI and bomb squad were at the scene trying to work out what or who was behind the explosions.
The mayor's office has set up an emergency hotline for friends and relatives on +1 617 635 4500.
The Federal Aviation Administration has created a no-fly zone over the area. Security at key sites in Washington DC and New York has been tightened.