One of the two bombs used in the Boston Marathon attacks used a pressure cooker, the FBI said in a Joint Intelligence Bulletin.
The second device, the agency said, was housed in a metal container, "but currently there is insufficient evidence to determine if it was also a pressure cooker," the bulletin said.
The alert also said the fuzing system and method of initiation for the two devices are unknown.
The two devices, which exploded near the finish line of the Boston Marathon on Monday, left three people dead and more than 180 injured. No suspects have been identified.