Investigators believe the lid of a pressure cooker found on a roof of a building near the scene of the Boston Marathon bombings is tied to the explosions that killed three people and wounded 183 on Monday afternoon.
Federal law enforcement revealed more details on the two bombs that went off 12 seconds apart near the marathon’s finish line.
The bomb held in the pressure cooker was hidden in a backpack, the FBI disclosed. Also, the device had fragments of nails, BBs and ball bearings. The second bomb was held in a metal container but investigators are uncertain of whether it was also a pressure cooker.
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 (about 1.6 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 probably detonated by timers. But the FBI said details of the detonating system are still unknown.
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Investigators are still confused about whether the bombings were an act of foreign or domestic terrorism.
"All of the talking heads that discuss this incident and incidents like it, if your experience and your expertise is Middle East terrorism; it has the hallmarks of al Qaeda or a Middle East group. If your experience is domestic groups and bombings that have occurred here, it has the hallmarks of a domestic terrorist like Eric Rudolph in the 1996 Atlanta Summer Olympics bombings," former FBI Assistant Director Tom Fuentes told CNN.
President Obama will be traveling to Boston on Thursday to address the city.
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(Photo: AP Photo/FBI)