The explosives at the Boston Marathon may have been made out of pressure cookers filled with nails and ball bearings, the FBI said.
The Associated Press originally reported the information, citing an unnamed person briefed on the probe.
Later Tuesday, FBI Special Agent in Charge Richard DesLauriers confirmed the information. He said that pieces of black nylon and fragments of ball bearings and nails were found and that authorities believe the explosives were placed in black duffel bags.
The FBI released photos of the bomb and fragments and bag remnants.
The FBI has taken over the lead on the investigation that President Obama declared was an act of terrorism.
The AP source said that investigators have some of the bomb components but did not yet know what was used to set off the explosives.
Pressure cooker explosives have been used in Afghanistan and other countries according to a July 2010 joint FBI and Homeland Security intelligence report.
One of the three devices used in the May 2010 Times Square attempted bombing was a pressure cooker, the intelligence report said.
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