The Black Hawk helicopter that collided with a passenger jet in Washington on Wednesday was on a training flight along a route core to a seldom-discussed military mission to evacuate senior officials to safety in the event of an attack on the U.S., officials say.

Diving crews are still searching the icy waters of the Potomac River looking to retrieve the bodies of all the people killed in this week’s plane and helicopter collision. Retired Black Hawk instructor pilot Martin Gottholm discusses the difficulties in flying at night for Black Hawk pilots and questions surrounding the altitudes of the aircrafts at the time.

The flight data recorder and cockpit voice recorder, known as the black boxes, may offer clues about what went wrong before the crash.