Incorrect altitude readings on the Army helicopter that collided with a passenger plane over Washington, D.C., in January contributed to the aircraft getting too close, but air traffic controllers warned about the hazards helicopter traffic presented years before the crash.

Three days of investigative hearings on the deadly midair collision over Washington in January began Wednesday, aiming to reveal new insights into what caused the crash between a passenger plane and an Army helicopter that killed 67 people.

The NTSB is holding three days of hearings to uncover insights into what caused the fatal crash between the passenger jet and the army helicopter over Ronald Reagan National Airport in January.