A black box of Flight MU5735 is found at the crash site of the plane in Tengxian County, Wuzhou City, south China's Guangxi Zhuang Autonomous Region, March 23, 2022. (Photo: CFP)

The Civil Aviation Administration of China (CAAC) on Wednesday issued a preliminary report on the deadly China Eastern Airlines Flight MU5735 crash in March, but the cause of the crash was not identified in the document.

The plane crashed into a mountainous area in the Guangxi Zhuang Autonomous Region at 14:21 on March 21, killing all 123 passengers and nine crew members aboard.

The update came 30 days after the incident.

The two "black boxes," a cockpit voice recorder and a flight data tracker, were recovered and are being analyzed in the hopes of unlocking the mystery behind the jet's rapid descent.

The devices "were severely damaged due to the impact, and data restoration and analysis work is still in progress," according to the report.

Investigations found the flight and cabin crews' qualifications "meet requirements," and the same was said of the maintenance staff, the CAAC said in a statement.

"The airworthiness certificate of the aircraft in the accident was valid," the statement said.

There was no report of fault before the flight, nor was there cargo declared as dangerous goods on the plane, said the CAAC, noting no abnormalities in navigation and monitoring equipment.

China Eastern previously said that the plane's captain and two co-pilots were not under suspicion.

The aircraft went down near Wuzhou in south China's Guangxi after losing contact with air traffic control.