Investigators in white jumpsuits have fanned out across the streets, rooftops and awnings around the site of Monday's Boston Marathon bombings. They're combing through debris, garbage cans and trash in search of clues to help determine who was behind the blasts that killed three people and injured more than 170. An official says the search has turned up a piece of circuit board and a pressure cooker lid that are believed to be parts of one of the bombs.