BOSTON (AP) -- FBI agents searched a suburban Boston apartment overnight and appealed to the public for amateur video and photos that might yield clues to who carried out the Boston Marathon bombing, while a doctor treating the wounded said one of the victims was maimed by what looked like ball bearings or BBs.
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. Three people were killed, including an 8-year-old boy, and more than 140 were wounded.
Federal investigators said no one had claimed responsibility for the bombings on one of the city's biggest civic holidays, Patriots Day. But the blasts raised the specter of another terrorist attack on U.S. soil.
President Barack Obama was careful not to use the words "terror" or "terrorism" as he spoke at the White House on Monday, but an administration official said the bombings were being treated as an act of terrorism.
"We will find out who did this. We'll find out why they did this," the president said. "Any responsible individuals, any responsible groups, will feel the full weight of justice."
Across the U.S., from Washington to Los Angeles, police tightened security, monitoring landmarks, government buildings, transit hubs and sporting events.
The FBI took charge of the investigation, converging on a home in the suburb of Revere on Monday night and appealing for any video, audio and photos taken by marathon spectators. Authorities gave no details on the search. Investigators were seen leaving a building there early Tuesday carrying brown paper bags, plastic trash bags and a duffel bag.
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 Tuesday initial evidence indicates that the attacks were not the work of suicide bombers.
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