Investigators say the crew of the helicopter that collided midair with an American Airlines jet near Washington D.C.’s Ronald Reagan National Airport might not have heard instructions from the air traffic controller to pass behind the plane.

The crew of an Army Black Hawk helicopter might not have heard key instructions from air traffic control before a collision with a passenger jet over the Potomac River on Jan. 29, the National Transportation Safety Board said Friday in its most detailed account of the moments before the crash that killed 67 people.

National Transportation Safety Board officials said Friday that a key transmission from Reagan National Airport’s air traffic control tower may not have been heard by Army Black Hawk helicopter pilots just before they were collided with an American Airlines regional jet last month,