Richard DesLauriers, the Boston FBI agent in charge of the investigation, said evidence collected so far at the scene includes parts of a pressure cooker, a circuit breaker, a battery with wires attached, and countless pieces of metal such as nails, ball bearings and buckshot-like BBs. Hospital doctors also turned over the pieces of shrapnel removed from the victims' bodies.
DesLauriers said the forensic evidence is being sent to the FBI laboratory in Quantico, Va., for analysis and to reconstruct the bombs and try to find out where the material in them might have originated. Pressure cooker explosives were found in the bomb attempt in New York's Times Square in 2010, and they have been used by terrorists in India, Nepal, Pakistan and Afghanistan.
The FBI agent urged anyone who was present at the crime scene at the time of the explosions and may have seen anyone carrying a heavy black duffel bag or backpack to contact authorities. He also asked for tips on anyone who may have talked about a bombing before the marathon.
"The person who did this was someone's friend, co-worker or neighbor," said DesLauriers. "Somebody knows who did this."
Details for this story were provided by law enforcement officials.