The first of the National Transportation Safety Board’s three days of investigative hearings is underway to help determine what caused the deadly midair collision on January 29 between an Army helicopter on a training mission and American Airlines flight 5342 landing at Ronald Reagan Washington National Airport.

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.