A federal law enforcement source with firsthand knowledge of the investigation told CNN that a lid to a pressure cooker thought to have been used in the Boston Marathon bombings had been found on a roof of a building near the scene.
While such clues may move the investigation forward, they did not reveal whether the attack was an act of domestic or foreign terrorism.
The two bombs, which exploded within 12 seconds of each other, killed three people and wounded 183.
One was housed in a pressure cooker hidden inside a backpack, the FBI said in a Joint Intelligence Bulletin.
The device also had fragments that may have included nails, BBs and ball bearings, the agency said.