"I was expecting my husband any minute," she said. "I don't know what this building is ... it just blew. Just a big bomb, a loud boom, and then glass everywhere. Something hit my head. I don't know what it was. I just ducked."
Runners who had not finished the race were diverted straight down Commonwealth Avenue and into a family meeting area, according to an emergency plan that had been in place.
The nearby Prudential Tower, the city's second-tallest building with an upscale shopping mall on the ground, was evacuated, along with the luxury Mandarin Oriental hotel, according to media reports.
Race day got started with 26 seconds of silence in honor of the victims of the December school shooting in Connecticut. A little more than 2 hours later, the lead runners passed the Mile 26 marker, which was decorated with the Newtown, Connecticut, seal and dedicated to the memory of those killed there.
The annual 26.2-mile (42-kilometer) marathon takes place on Patriot's Day, a state holiday that celebrates the evacuation of Boston by the British in the American Revolution.