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.

Investigators searching for wreckage from last week’s deadly midair collision between a US military Black Hawk helicopter and an American Airlines plane near Washington, DC, conducted an overnight scan of the Potomac River where the aircraft fell using advanced imaging technology to map the riverbed and locate debris.