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 crash was the deadliest U.S. air crash in nearly 25 years, claiming the lives of all 67 people on board both aircraft.

A key safety system was turned off in a US army helicopter when it crashed into an American Airlines flight last week, killing 67 people.

The NTSB says pilots of the airplane that crashed into a military helicopter in Washington, DC tried to quickly climb a second before the crash. The American Airlines flight crashed into a Blackhawk helicopter last month killing 67 people.

The National Transportation Security Board is set to give an update Friday on a midair collision between a helicopter and passenger plane near Washington, D.C. last month.