The FBI took charge of the investigation, converging on a home in the suburb of Revere last night. Authorities gave no details on the search. Investigators were seen leaving a building there early today carrying brown paper bags, plastic trash bags and a duffel bag.
They also repeatedly appealed for any video, audio and photos taken by marathon spectators, even images that people think 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 gathered a large number of surveillance tapes from businesses and intend to go through them frame by frame.
Investigators refused to give any specifics on the bombs and say, for example, where they might have been hidden or whether they were packed with shrapnel for maximum carnage, as is often the case in terror bombings overseas.
But Dr. Stephen Epstein of the emergency medicine department at Beth Israel Deaconess Medical Center said he saw an X-ray of one victim's leg that had "what appears to be small, uniform, round objects throughout it — similar in the appearance to BBs." He said it remained to be determined what exactly the objects were.
A European security official said today initial evidence indicates that the attacks were not the work of suicide bombers.
"So far, investigators believe it was not the work of suicide bombers, but it is still too early to rule it out completely," said the official, who spoke from the United States on condition of anonymity because he was not authorized to speak about the U.S. investigation.
The Pakistani Taliban, which has threatened attacks in the United States because of its support for the Pakistani government, today denied any role in the bombings.