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.

The pilots of a Black Hawk helicopter that collided with a passenger jet near Washington Reagan National Airport in January likely didn’t know how high they were flying or how close they were to an airliner before the deadly crash,