U.S. authorities restricted helicopter flights near Reagan Washington National Airport on Friday, after a midair collision between an American Airlines passenger jet and a military helicopter killed 67 people this week.

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.

Investigators found the recording devices, or black boxes, from the American Airlines plane and Black Hawk helicopter that crashed on Wednesday.