The army helicopter that crashed into a passenger jet over Washington DC was flying too high, investigators have concluded.

Officials with the National Transportation Safety Board (NTSB) opened a three-day hearing into the Jan 30 incident, in which 67 people were killed, on Wednesday.

The review found that the pilots of the Black Hawk helicopter that crashed into the American Airlines plane from Wichita, Kansas, did not hear a warning from air traffic controllers to pass behind the jet. Some critical instruments were also not working, investigators said.

“There is a possibility that what the crew saw was very different than what the true altitude was,” Jennifer Homendy, the NTSB chairwoman, told the hearing.

Air safety experts have long warned the skies around Ronald Reagan Washington National Airport in the capital are too busy.

The January incident was the nation’s deadliest plane crash since November 2001.