At least three people, including an eight-year-old boy, were killed when two bombs exploded in the crowded streets near the finish line of the Boston Marathon.
More than 140 other people were injured.
As the FBI took charge of the investigation, authorities shed no light on a motive or who may have carried out the bombings, and police said they had no suspects in custody. Officials in Washington said there was no immediate claim of responsibility.
As many as two unexploded bombs were also found near the end of the 26.2-mile (42km) 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.
One runner, a Rhode Island state police officer, said he saw at least two dozen people with very serious injuries, including missing limbs.
A no-fly zone was put into force in the area.
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About two hours after the winners crossed the finish line, there was a loud explosion on the north side of Boylston Street, just before the photo bridge that marks the line. Another explosion could be heard a few seconds later.
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The Boston Marathon said that bombs caused the two explosions and that organisers were working with authorities to determine what happened.
Competitors and race volunteers were crying as they fled the chaos. Bloody spectators were being carried to the medical tent that had been set up to care for fatigued runners.
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President Barack Obama gave a live TV address from the White House, where he paid tribute to the efforts of the emergency services.
“We salute all those who assisted in responding so quickly and professionally to this tragedy,” he said.
The President said people “should not jump to conclusions” about possible motives for the attack, but he vowed: “We will find out who did this and we will hold them accountable.”
The administration said it is in contact with state and local authorities and the president directed his administration to provide whatever assistance is necessary in the investigation and response.
Speculation that a fire at the city’s JFK library might be related to the marathon attack has now been ruled out. Authorities now say it was a standard fire.The annual marathon takes place on Patriot’s Day, a state holiday that celebrates the evacuation of Boston by the British in the American Revolution.