CROWDED SKIES, UNDERSTAFFING, AND A FATAL MISTAKE: The investigation has just begun, but a clearer picture is already emerging of the cause of Wednesday night’s midair collision between an American Airlines regional jet and a U.S. Army Black Hawk helicopter that killed all 67 people on both aircraft.

To understand the proximate cause of the accident — that is to say the last of many factors that led to the deadly accident — you have to be able to picture the layout of Washington’s Reagan National Airport located on the west shore of the Potomac River. The passenger jet approaching from the south, curved to the right, moving over land to align with Runway 33, a shorter runway that cuts diagonally across the main north-south runway. The Army Black Hawk was proceeding south on a night proficiency exercise along the river’s eastern edge, in an area designated for helicopter traffic.