National Transportation Safety Board officials said Friday that a key transmission from Reagan National Airport’s air traffic control tower may not have been heard by Army Black Hawk helicopter pilots just before they were collided with an American Airlines regional jet last month,

That transmission was interrupted — it was stepped on,” said Jennifer Homendy, chair of the National Transportation Safety Board.

The National Transportation Safety Board said the helicopter’s cockpit voice recorder didn’t capture key directions from air-traffic control and is investigating whether the crew might have seen a different altitude on the flight instruments.