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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 wasn’t enough evidence to determine whether it was also in a pressure cooker, the FBI said.

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The discovery of the lid could yield vital clues as to the origin of the bomb.

Nails, scraps found
Scraps of at least one pressure cooker, nails and nylon bags found at the scene were sent to the FBI’s national laboratory at Quantico in Virginia, where technicians would try to reconstruct the devices, the federal agent leading the investigation said on Tuesday.

The pieces recovered so far suggest the devices could carry six 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.

Meanwhile, investigators are sifting through more than 2,000 tips and a mass of digital photos and video clips, but are asking for public help in providing additional leads and images.

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