Salvage crews worked on the Potomac River on Thursday and could be seen pulling out pieces of the Army Black Hawk helicopter that crashed with a plane.

Last month, an American Airlines passenger jet and an Army helicopter collided as the plane prepared to land at Washington D.C.’s Ronald Reagan National Airport.

National Transportation Safety Board Chairwoman Jennifer Homendy told reporters that the recording from the Black Hawk helicopter cockpit suggested an incomplete radio transmission may have left the crew without understanding how it should shift position just before the Jan.