As decoding and analysis work for the two "black boxes" recovered from the MU5735 crash is ongoing, other avenues for investigation are also progressing, an official with the Civil Aviation Administration of China said on Monday.

Two black boxes were recovered and sent to Beijing for decoding and analysis - one on Sunday and the other on Wednesday, Zhu Tao, head of the CAAC safety office, said at a news conference in Wuzhou, Guangxi Zhuang autonomous region.

The flight data recorder can record 25 hours of around 1,000 types of flight data, including altitude, speed, direction and how the crew operated the aircraft. The other, the cockpit voice recorder, could record for two to three hours anything said in the cockpit as well as ambient sounds.

Apart from analyzing black boxes, the investigation team is also collecting other evidence such as aircraft debris, accident-related video clips and witnesses' accounts, Zhu said.

"To restore the truth, it is insufficient to rely only on data from the black boxes in an air crash accident investigation," he said.

Shu Ping, director of the China Academy of Civil Aviation Science and Technology's Aviation Safety Institute, said the investigation for an air crash involves not only black boxes but also information collected at the crash site and experimental verification.

For example, he said, if at the crash site a tree branch was seen to be pulled inside the engine, investigators can determine that the engine was running before hitting the ground. But the flight data recorder would be needed to establish the speed of the engine.

"Many objects at the crash site could be used to analyze the situation, such as the condition of the engine system," he told CAAC News.

Flight MU5735 crashed with 123 passengers and nine crew members aboard into a mountain in the Guangxi Zhuang autonomous region at 2:21 pm on March 21. All those onboard were confirmed dead.