The two bombs detonated at the Boston Marathon exploded within 12 seconds of each other and 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.
The second bomb was also housed in a metal container, but there’s not enough evidence to determine whether it was also in a pressure cooker, the FBI said.
The U.S. government has warned federal agencies in the past that pressure cookers — airtight pots used to quickly cook or preserve foods — have been packed with explosives and shrapnel and detonated with blasting caps.
Photos obtained by CNN showed the remains of a pressure cooker found at the scene, along with a shredded black backpack and what appear to be metal pellets or ball bearings.
Scraps of at least one pressure cooker, nails and nylon bags found at the scene are being sent to the FBI’s national laboratory in Virginia, where technicians will try to reconstruct the devices, the federal agent leading the investigation said Tuesday.
The pieces recovered so far suggest the devices could carry 6 liters (1.5 gallons) each, a Boston law enforcement source said. The parts found also include a partial circuit board, which would be used to detonate a device, as well as the lid found on the rooftop.
A law enforcement official said Monday’s bombs were likely detonated by timers. But the FBI said details of the detonating system are still unknown.