From The New York Times:
Clues emerging from the moments before the deadly collision Wednesday night between an Army helicopter and an American Airlines passenger jet suggest that multiple layers of the country’s aviation safety apparatus failed, according to flight recordings, a preliminary internal report from the Federal Aviation Administration, interviews with current and former air traffic controllers and others briefed on the matter.
The helicopter flew outside its approved flight path. The American Airlines pilots most likely did not see the helicopter close by as they made a turn toward the runway. And the air traffic controller, who was juggling two jobs at the same time, was unable to keep the helicopter and the plane separated. An F.A.A. spokesman said the agency could not comment on the ongoing investigation, which is being led by the National Transportation Safety Board.