Investigators say the crew of the helicopter that collided midair with an American Airlines jet near Washington D.

The Army helicopter that crashed into an American Airlines jet in Washington, D.C. may not have heard important instructions from air traffic controllers, according to the National Transportation Safety Board.

The National Transportation Safety Board said the helicopter’s cockpit voice recorder didn’t capture key directions from air-traffic control and is investigating whether the crew might have seen a different altitude on the flight instruments.