According to the National Transportation Safety Board, Black Hawk pilots might not have received an important instruction from air traffic control to maneuver behind the American Airlines aircraft just before their collision.

Seventeen seconds before the deadly Jan. 29 crash, which killed all 67 people aboard both flights, the Black Hawk was directed to pass behind the passenger jet, NTSB Chairwoman Jennifer Homendy said at a news conference.

“They may have not received the ‘pass behind the’ because they were communicating with air traffic control,” Homendy explained.

The Black Hawk pilots likely “stepped on” the communication by pressing the mic in an attempt to communicate back to air traffic control.

There was also an apparent discrepancy between two of the Army pilots about what altitude they were flying at, according to investigators.

At one point before the collision, the helicopter’s pilot announced that it was at 300 feet, but the instructor pilot said the helicopter was at 400 feet, Homendy said.