Information about Boston bombs emerges

Investigators combing the site of the Boston Marathon bombing have recovered a piece of a circuit board they believe was part of one explosive device and the lid of a pressure cooker blown onto a rooftop, according to federal law enforcement officials.

The additional pieces of the bombs that authorities believe were fashioned from ordinary pressure cookers packed with nails, ball bearings and other shrapnel may ease the task of tracing the manufacturer and possibly the place where the components were purchased, clues that might lead to the bomber. The first days of the FBI-led probe of the bombing are providing a clearer picture of the devices that killed three people and injured more than 170 others in Boston.

Investigators also found scraps of black nylon at the blast site that suggest the bombs were hidden in a black duffel bag or backpack. With the investigation "in its infancy," Richard DesLauriers, the special agent in charge of the Boston FBI office, yesterday asked members of the public to search their memories for any recollection of someone who may have talked about the marathon as a target or who showed an interest in explosives.

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Investigators continue to work the scene of two bomb explosions at the finish line of the marathon that killed 3 people and injured over one hundred more. Photograph: Darren McCollester/Getty Images

The unexplained sound of a blast in a remote area might have signaled a test run, he said. "The person who did this was someone's friend, neighbor or co-worker," DesLauriers said at a news conference yesterday. "Someone knows who did this."

He also sought witnesses at the marathon on race day for information or photos of anyone carrying a heavy dark bag. Law enforcement officials said they had no suspects in the bombing, one of the highest-profile acts of terror in the US since the September 11th attacks on New York City and Washington in 2001. No group or organization claimed responsibility, and US politicians and intelligence officials have said there were no indications of a plot beforehand.

Additional reporting: Agencies