BOSTON - Federal investigators said no one had claimed responsibility for the bombings, which took place at the world's best-known distance race, held every year on one of Boston's biggest 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.
Boston police and firefighter unions announced a $50,000 reward for information leading to arrests in the bombing.
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.
"There has to be hundreds, if not thousands, of photos and videos" that might help investigators, state police Col. Timothy Alben said.
Boston Police Commissioner Edward Davis said investigators also gathered a large number of surveillance tapes from businesses in the area and intend to go through the videos frame by frame.
"This is probably one of the most photographed areas in the country yesterday," he said.
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 the official said it is possible the man was simply running away to protect himself from the blast, as many others did.