Military helicopter traffic over Washington was a longtime worry among aviators operating around the airport where an Army helicopter and commercial airliner collided this year, killing 67 people, federal investigators heard during testimony on Thursday.

The hearings in Washington will involve NTSB board members, investigators and witnesses. Panels will focus on military helicopter routes in the Washington area, collision avoidance technology and training for air traffic controllers at Ronald Reagan National Airport, among other subjects.

Investigators have found that a helicopter involved in a deadly crash with a passenger plane over Washington was flying above its height limit. The helicopter's altitude-measuring instrument was also inaccurate.