DEVELOPING: Investigators have arrested a suspect in the Boston Marathon bombing after reviewing photos and video from Monday's event, a federal source confirmed to FoxNews.com.

The suspect, who is expected to appear in federal court in Boston later today, has not yet been named by authorities. He was seen on surveillance footage that showed a young man carrying – and perhaps dropping – a black bag at the second bombing scene at the finish line of Monday's race, where two bombs exploded, killing three and injuring 176. The camera at a Boylston Street Lord & Taylor directly across the street from one of the blasts, has given investigators clear video of the area, according to the unnamed official.

“The camera from Lord & Taylor is the best source of video so far,” said Dot Joyce, a spokeswoman for Boston Mayor Thomas M. Menino told the Boston Globe. “All I know is that they are making progress.”

Earlier, Fox News learned that the circuit board suspected of being used to detonate the at least one of the bomb has been recovered, and that FBI investigators continue to scan and analyze the cell phone tower records to identify positive hits for signs of calls that may have triggered both explosions remotely.

According to a FBI and Department of Homeland Security bulletin, the evidence at this time supports just one pressure cooker used. Shrapnel in that device included nails, BBs and ball bearings.  The main charge - possibly low explosive - has not been identified.  The other device "was also housed in a metal container, but currently there is insufficient evidence to determine if it was also a pressure cooker. The fuse system and method of initiation for the two devices are unknown at this time."

Pressure cooker bombs have been used in high-profile bombings in India, Pakistan and Afghanistan, and have been touted in the Al Qaeda in Yemen's online propaganda magazine Inspire and in the "The Anarchist Cookbook."