GERRY Carr (54) from Blanchardstown, Dublin, is in Boston with the Clonliffe Harriers Athletics Club.
He said: "I'd just come through the finishing line, about a 100 yards through the finish line. I just heard this explosion behind and it took my attention.
"I would have been gone through the line about 20 seconds I'd say.
"I looked back and there was a big puff of smoke right on the finish line and within 30 seconds of it there was another explosion, from what I could see, somewhere in the same area."
"All hell broke loose. There was immediate panic on the street, people just running away in one direction and the police moving in the other direction towards where the bomb had exploded.
"There would have been thousands of people in the area -- spectators, you'd have marathon staff on the finish line, and you would have people finishing the race. Not all the Irish people are back to the hotel here yet. I think there are three still to come in.
"There are transport problems getting from there. We're about a mile-and-a-half from where the race finish is.
"The subway in that area is not running and you can't get taxis either.
"I don't think they would have been due at the finish line at the time of the explosion. They should still have been half an hour away when it went off," he added.