A preliminary analysis of the flight data and voice recorder on board a Black Hawk helicopter leading up to the collision with a commercial flight over Washington, DC, on January 29, indicated the helicopter’s altimeter may have been inaccurate and the pilots may not have heard some calls from the Reagan National Airport control tower,

The Army pilots were juggling dark skies, low altitude, a busy airspace and a cockpit without certain traffic detectors before the helicopter’s midair crash with a regional passenger jet.

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.

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.