The crew of a US Army helicopter may have had faulty altitude readings and not heard a key instruction from air-traffic controllers moments before colliding with an American Airlines Group Inc. regional jet near Washington in late January,

From the perspective of the helicopter cockpit, the lights of the incoming American Airlines jet could have also been obscured by the glow of the city skyline, pilots said.

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.