Runner Stephen Purdy said he did not know there had been any explosions until he phoned his wife after completing the marathon.
"I just went back to the hostel and got showered and changed and phoned my wife - she was in tears and my son answered," he told the BBC.
"She was just so glad to hear from me, she told me then there had been an explosion.
"So it is a bit of a shock."
The two explosions, which went off near near the finishing line with the marathon clock at 4:09:44, have 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".
At least 17 people are critically wounded, officials say, and the injuries include several amputations.
One of the dead was an eight-year-old boy, US media said.
A third explosion at the nearby JFK Library was also confirmed by Boston Police.
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