The Boston marathon bombs were explosives packed in six-litre pressure cookers and placed in black duffel bags, according to a source close to the investigation.
They said the explosives were placed on the ground and contained shards of metal, nails and ball bearings. They did not want to be named.
The source said police have some of the bomb components but did not yet know what was used to set off the explosives.
At least three people, including an eight-year-old boy, were killed when two bombs exploded in the crowded streets near the finish line of the Boston Marathon.
More than 150 other people were injured.
One of the bombs contained shards of metal and ball bearings, and another contained nails, the source said.
Pressure cooker explosives have been used in Afghanistan, India, Nepal and Pakistan, 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.
"Placed carefully, such devices provide little or no indication of an impending attack," the report said.
Typically, these bombs have an initiator, switch and explosive charge, according to the Homeland Security Department.
Federal and local investigators swarmed the city to try to hunt down those responsible for a pair of explosions that also caused injuries to at least 150 other people, at least 17 of whom remain in a critical condition.