An air traffic controller at Reagan National Airport reportedly left early on Wednesday before the deadly collision between an American Airlines flight and a military helicopter.

The National Transportation Safety Board has interviewed the air-traffic controller who was working at the time of the midair collision, board member Todd Inman said, adding that the interviews have been ongoing throughout the day.

A single controller was left to handle the air traffic of planes and helicopters flying over the airspace when two people would typically be monitoring both flight paths