An American Airlines jet and an Army helicopter collided over Washington, D.C., Wednesday night. The number of casualties is unclear and a search-and-rescue mission is ongoing.

An airspace cluttered with passenger planes and military aircraft. A history of near-crashes. And a growing shortage of air traffic controllers available to manage it all. Some experts, politicians and airport managers have been warning for years of the risks posed by the crowded airspace and volume of flights at Ronald Reagan Washington National Airport (DCA).

Questions remain over the mid-air collision between a jet, which was carrying 64 people, including the crew, and a military helicopter carrying three soldiers