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.

U.S. authorities said on Thursday it was not yet clear why a regional jet and a U.S. Army helicopter collided at a Washington airport, killing 67 people in the deadliest U.S. air disaster in more than 20 years.