A man from Belfast who was taking part in the Boston Marathon has described the pandemonium caused by the attack.
Michael Roberts, 61, had finished the race and was in the medical tent close to the explosion when the bombs started to go off.
"There was two bangs and I said 'I'm from Belfast – I know that sound – it's a bomb'," he told UTV.
"Then within a few minutes there was pandemonium in the medical tent.
"It was all over in a flash. It reminded me of Belfast. I recognised the sound.
"I am fine now but these things take their toll."
The two explosions, which went off with the marathon clock at 4:09:44, have left three people dead and at least 140 injured.
One of the dead was an eight-year-old boy, US media said.
At least 17 people are critically wounded, officials say, and the injuries include several amputations.
The FBI has taken over co-ordination of what it described as a "potential terrorist inquiry".
A third explosion at the nearby JFK Library was also confirmed by Boston Police.