US investigators believe someone in the cockpit deliberately crashed a China Eastern flight that suddenly plunged to the ground in southern China in March, the Wall Street Journal has reported.

The Boeing 737-800 was on its way from Kunming to Guangzhou on March 21 when it dropped from its cruising altitude of 29,000 feet into a mountainside, killing all 132 people on board. It was mainland China’s worst aviation disaster in nearly 30 years.

The flight data recorders recovered from the crash site were sent to the United States for analysis and show that someone – possibly a pilot or someone who had forced their way into the cockpit – input orders to send the aircraft into a nosedive.

“The plane did what it was told to do by someone in the cockpit,” a person familiar with the preliminary assessment by experts on the US National Transportation Safety Board told the Journal.

The pilots did not respond to repeated calls from air traffic controllers and nearby planes during the rapid descent, authorities have said. One source told the Reuters news agency that investigators were looking at whether the crash was a “voluntary” act.

Screenshots of the Wall Street Journal story appeared to have been censored on Weibo, China’s Twitter-like platform, and messaging app Wechat on Wednesday morning.

The Civil Aviation Administration of China said on April 11, in response to internet rumours of a deliberate crash, that the speculation had “gravely misled the public” and “interfered with the accident investigation work”.

Boeing and the NTSB declined to comment to news agencies and referred queries to Chinese regulators. China Eastern did not immediately respond to requests for comment.

According to a report from Boeing, investigators found no evidence of “anything abnormal,” China’s Civil Aviation Administration (CAAC) said in April.

In a statement, the CAAC said staff had met safety requirements before takeoff; the plane was not carrying dangerous goods and did not appear to have run into bad weather, although the agency said a full investigation could take two or more years.

Deliberate crashes are exceptionally rare.

Experts noted the latest hypothesis left open whether the action stemmed from one pilot acting alone or was the result of a struggle or intrusion, but sources stressed nothing had been confirmed.

In March 2015, a German wings co-pilot deliberately flew an Airbus A320 into a French mountainside, killing all 150 on board. (Al Jazeera)

•PHOTO:Rescue workers searched muddy hillsides after the China Eastern Airlines Boeing 737-800 plane crashed in March [File: Carlos Garcia Rawlins/Reuters]