On their own, they're simple household items. Pressure cookers. Nails. Nylon bags. But combine them with explosives, and you can get the kind of bombs that investigators now suspect were used to turn the finish line of the Boston Marathon into a bloody nightmare.

Scraps of them found at the scene of the twin bombings are being sent to the FBI's national laboratory, where technicians will try to reconstruct the devices, the federal agent leading the investigation said Tuesday. Meanwhile, investigators combed through a digital mountain of photos and video clips and asked the public for help finding who planted them at two points on Boylston Street.

"Someone knows who did this," said Rick DesLauriers, the special agent in charge of the FBI's Boston office. "Cooperation from the community will play a crucial role in this investigation."

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