The China Eastern jetliner was flying between the cities of Kunming and Guangzhou on March 21 when it nosedived into a mountainside, disintegrating on impact and killing all 132 people on board.

The cause of the disaster, China’s deadliest plane crash in more than 30 years, is not yet known.

“NTSB investigators are assisting the Civil Aviation Administration of China with the download of the cockpit voice recorder from China Eastern Flight 5735 in our lab in Washington,” spokesman Peter Knudson told AFP in an email.

He declined to give any details of this work.

The other black box from the plane, the flight data recorder, has also been found, and it too could help solve the mystery of what went wrong.

This recorder contains crucial information such as the speed of the aircraft, its altitude and heading. It was found March 23 and sent to Beijing for analysis.

While parts of this recorder were seriously damaged, the data storage unit appears relatively intact, Zhu Tao, director of safety at the Civil Aviation Administration of China (CAAC), told reporters last weekend.

The aircraft went down near Wuzhou in southern China after losing contact with air traffic control.

Tracking website FlightRadar24 showed the jet sharply dropped from an altitude of 29,100 to 7,850 feet in just over a minute.

After a brief upswing, it dropped again to 3,225 feet, the tracker said. There is no data for the flight after 2.22pm.