"They can give me a cavity search right now and I'd be perfectly happy," said Daniel Wood, a video producer from New York City who was waiting for a train.
Two bombs blew up seconds apart Monday at the finish line of one of the world's most storied races, tearing off limbs and leaving the streets spattered with blood and strewn with broken glass. The dead included an 8-year-old boy.
Federal investigators said no one had claimed responsibility for the bombings, which took place on one of the city's biggest civic holidays, Patriots Day.
"We will go to the ends of the Earth to identify the subject or subjects who are responsible for this despicable crime, and we will do everything we can to bring them to justice," said Richard DesLauriers, FBI agent in charge in Boston.
He said investigators had received "voluminous tips" and were interviewing witnesses and analyzing the crime scene.
Gov. Deval Patrick said that contrary to earlier reports, no unexploded bombs were found. He said the only explosives were the ones that went off.
FBI agents searched an apartment in the Boston suburb of Revere overnight, and investigators were seen leaving with brown paper bags, plastic trash bags and a duffel bag. But it was unclear whether the tenant had anything to do with the attack.
A law enforcement official who spoke on condition of anonymity because he was not authorized to release details of the investigation said the man had been tackled by a bystander, then police, as he ran from the scene of the explosions.
But he said it is possible the man was simply running away to protect himself from the blast, as many others did.
At a news conference, police and federal agents repeatedly appealed for any video, audio and photos taken by marathon spectators, even images that people might not think are significant.