Police boats combed the banks of the Potomac River on Friday, slowly scanning the shoreline in the rain as investigators sought clues into the midair collision that killed 67 people and raised questions about air traffic safety around the nation’s capital.

It's one of the most important pieces of forensic evidence following a plane crash: The so-called “black box." There are actually two of these remarkably sturdy devices: the cockpit voice recorder and the flight data recorder.