Air traffic controller failed to notify commercial plane crew about approaching Army helicopter before Washington D.C. collision that killed 67 people, FAA official says.

It was the first acknowledgment by the Federal Aviation Administration of a possible error by the controller in the moments before the collision that claimed 67 lives.

The National Transportation Safety Board will question witnesses Thursday about air traffic control training, a key part of the investigation into January’s deadly midair collision at Ronald Reagan Washington National Airport.