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).

A military helicopter and passenger jet collided midair on Wednesday night near Reagan Washington National Airport, according to the Federal Aviation Administration. Here's what we know.