Authorities arrest suspect in Boston bombings, source says

Authorities arrested a suspect Wednesday in the fatal bombings at the Boston Marathon, a federal law enforcement source told CNN's Fran Townsend.

Details about the suspect's identity and motive were not immediately available. News of the arrest was first reported by CNN.

Investigators identified the suspect based on two videos taken before Monday's attack, the federal law enforcement source told Townsend.

One of the videos was surveillance footage taken at the nearby Lord & Taylor department store, the other was shot by a Boston television station, a source who has has been briefed on the investigation exclusively told CNN's John King.

The development came after a chaotic day in which investigators revealed more details about the makeup of the bombs and apparently unrelated scares over letters containing ricin gripped the nation's capital.

The bombs exploded 12 seconds apart near the finish line of the Boston Marathon, killing three people and wounding about 180 others.

One of the bombs 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 it was not clear whether it too was in a pressure cooker, the FBI said.

The U.S. government has warned federal agencies in the past that terrorists could turn pressure cookers into bombs by packing them with explosives and shrapnel and detonating them with blasting caps.

The bombs

Photos obtained by CNN show 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 were sent to the FBI's national laboratory in Virginia, where technicians will try to reconstruct the devices, the agent leading the investigation said Tuesday.

The pieces suggest each of the devices was 6 liters (about 1.6 gallons) in volume, a Boston law enforcement source said. The recovered parts include part of a circuit board, which might have been used to detonate a device.

A law enforcement official said Monday's bombs were probably detonated by timers. But the FBI said details of the detonating system were unknown.

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