The two bombs, which exploded within 12 seconds of each other at the marathon finish line Monday, killed three people and wounded 183, according to latest reports.
One of the bombs was housed in a pressure cooker hidden inside a backpack, the Federal Bureau of Investigation (FBI) said in a Joint Intelligence Bulletin.
The device also had fragments that may have included nails 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.
Discovery of the lid could yield vital clues as to the origin of the bomb, CNN said, citing a federal law enforcement source with firsthand knowledge of the investigation.
While such clues move the investigation forward, even for seasoned investigators, the theories run the gamut on whether Monday's attack was an act of domestic or foreign terrorism, the channel said.
CNN said photos obtained by it 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 at Quantico, 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 litres each. 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.
Monday's bombs were likely detonated by timers, CNN said citing a law enforcement official. But the FBI said details of the detonating system are still unknown.
Meanwhile, investigators are sifting through more than 2,000 tips and a mass of digital photos and video clips, but are asking for the public's help in providing additional leads and images.
