D.C. mid-air collision is the first U.S. airline crash since 2009.

Wednesday’s mid-air collision in Washington D-C was the first commercial airline crash in the U-S in nearly 16 years, when a Colgan regional jet crashed into a neighborhood outside of Buffalo, New York (in 2009). That crash killed 49 passengers and crew members, and exposed dangerous pilot training practices, which have since been banned. Investigators will be looking into whether air traffic control gave ample warning to both the Black Hawk helicopter and the American Airlines flight and if their onboard warning systems were even working.