Via LA Times:

Two suspects in the Boston Marathon bombing have been singled out in surveillance videos of the scene, sources told the Los Angeles Times on Wednesday.
U.S. counter-terrorism and law enforcement officials told The Times that the suspects in the Boston bombings were seen in a department store video that caught images of a man leaving a backpack near the finish line.

A second federal official said he has been briefed that authorities believe a second video or photo showed “two men with two backpacks.”

A counter-terrorism official said the suspects were picked out because they were carrying black backpacks, as well as because of their demeanor and how they reacted to the bomb blasts. That official said authorities do not yet know the identities of those in the videos.

Media personnel were gathering at the Boston federal courthouse Wednesday afternoon in anticipation of a possible arrest.