Wednesday night’s fatal crash at Ronald Reagan Washington National Airport involving a commercial airplane and a Black Hawk military helicopter has drawn renewed attention to air traffic controllers and what role they may have played in the accident that is believed to have killed 67 people.

Democrats previously hit out at the president's federal hiring freeze for hindering the "number one safety issue" in aviation.

Amid increased concerns about aviation safety, here's how many air traffic controllers there are in the U.S., and how the hiring process works.