NANNING -- Rescuers are making all-out efforts to retrieve the black boxes of a passenger plane with 132 people aboard that crashed in South China's Guangxi Zhuang autonomous region Monday afternoon, an official with the Civil Aviation Administration of China said Tuesday night.

The plane was severely damaged and the investigation is very difficult, Zhu Tao, head of CAAC aviation safety office, told a press briefing.

At present, the investigation team is carrying out a full probe in accordance with the procedures, with rescuers exploring the crash site and going all out to search the black boxes, Zhu said.

The team is also thoroughly investigating other aspects, including flight, maintenance, air traffic control, meteorology, aircraft design and manufacturing, Zhu said.

Based on known information, the authorities do not yet have a clear clue to the cause of the passenger plane crash, the official said.