As part of that assistance, the plane’s cockpit voice recorder is being downloaded and analyzed at a U.S. lab in Washington, federal officials said Friday.
Investigators hope the recording will explain why the plane went into a nosedive from about 8,800 meters (29,000 feet) over a mountainous region in southeastern China.
Chinese officials have said that air traffic controllers were unable to get a response from the pilots while the plane was descending.
The cockpit voice recorder would pick up voices and other sounds from microphones worn by the pilots and another stationed over their heads.
Searchers also recovered the plane’s flight-data recorder, which constantly captures speed, altitude, heading and other information and the performance of key systems on the aircraft, but that recorder was not being evaluated in Washington on Friday.
The NTSB said its investigators will limit contact with people outside the investigation so that they can start their work immediately without going through a quarantine period.
The plane that crashed was not a 737 Max, a newer model that was temporarily grounded worldwide following two deadly crashes in Indonesia and Ethiopia.
The impact caused by the crash in China created a 20-meter- (65-foot-) deep crater, set off a fire in the surrounding forest and smashed the plane into small parts scattered over a wide area, some of them buried underground. More than 49,000 pieces of debris have been recovered, along with some human remains and personal items.
A Chinese aviation safety official said a preliminary investigation report would be completed within 30 days of the crash.
Flight MU5735 with 123 passengers and nine crew members was headed from the southwestern city of Kunming, the capital of Yunnan province, to Guangzhou, a major city and export manufacturing hub near Hong Kong in southeastern China.
The 737-800 has an excellent safety record and the Chinese airline industry has had relatively few mishaps in recent years.
Before last month’s accident, the last fatal crash of a Chinese airliner occurred in August 2010, when an Embraer ERJ 190-100 operated by Henan Airlines hit the ground short of the runway in the northeastern city of Yichun and caught fire, killing 44 people. Investigators blamed pilot error.
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