Items found that may have caused the bomb in The Boston Marathon

Investigators have several pieces of evidence believed to be part of the homemade explosive devices used in the Boston Marathon bombing on Monday.

Nails, wires, pressure cooker, a shredded black backpack, pieces of shrapnel, a battery, and a partial circuit board.

A law enforcement official told CNN that the bombs were likely detonated by timers connected to the circuits and batteries, but the FBI said they haven't figured out the detonating system or what exactly set them off. The bombing was crucial injuring about 140 persons.