The crew of the Army helicopter that hit a jet may have had inaccurate altitude readings and may not have heard air traffic controllers' instructions, officials say.

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.

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.