NTSB investigators found Friday that the aircraft involved in the mid-air collision over DCA lacked certain collision avoidance systems

On day 1 of the NTSB’s hearings on the deadly crash, investigators revealed the helicopter pilots were likely unaware of how close they were to the plane before the collision.

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.

The head of the National Transportation Safety Board sharply criticized U.S. officials for failing to act before the January 29 collision of a U.S. Army helicopter with an American Airlines regional jet in Washington that killed 67 people.