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.

The National Transportation Safety Board is also studying whether the pilots of the Black Hawk in last month’s collision were seeing accurate data about the helicopter’s altitude.

A Black Hawk helicopter crew may not have heard key instructions from air traffic control before it struck an American Airlines jet near Washington DC a fortnight ago, according to black box data.