An air traffic controller failed to notify the crew of a commercial plane that an Army helicopter was moving toward the aircraft before the collision in Washington, D.C., in January that killed 67 people, a Federal Aviation Administration official said on Thursday.

The FAA’s first acknowledgment of a possible error in the control tower at Ronald Reagan National Airport just before the fatal collision came on the second day of National Transportation Safety Board hearings looking into what led to the midair crash.

Officials testified at the hearing about a high number of takeoffs and landings at the airport that, in addition to heavy helicopter traffic, led to air traffic controllers testing the limits of safety, according to The Washington Post.

Among the methods used the night of the crash were the frequent use of allowing helicopter crews to use their own judgment and skills to avoid other air traffic and the use of runway 33, which is shorter and requires jets arriving from the south to leave the usual flight path and travel further east, the outlet noted.

at the airport has been a problem since the crash, according to NTSB investigator Brian Soper, who said that the latest FAA data shows that while there are 26 controllers assigned to the tower, only 19 are available to work.