As search and rescue operations of the crashed MU5735 are nearly completed, the key task now is to find out the cause of the fatal accident and a preliminary report will be completed within 30 days of the accident, a senior civil aviation safety official said on Thursday.

"Both flight recorders, or black boxes, from the aircraft was recovered within seven days after the crash, and the identification of all passengers was confirmed within eight days. Currently, the key search and rescue operations are almost concluded," Zhu Tao, head of the Civil Aviation Administration of China's aviation safety office, said at a news conference held in Wuzhou of Guangxi Zhuang autonomous region.

China Eastern Airlines Flight MU5735 left the southwestern city of Kunming at 1:11 pm on March 21 with 123 passengers and nine crew members onboard. Air traffic controllers lost track of the plane over Wuzhou at 2:21 pm. All people onboard were confirmed dead on Saturday.

Zhu said rescuers found more than 49,000 pieces of debris of the aircraft, including key parts of the engines and wings, by Wednesday. They are being disinfected, categorized and labeled for investigation, which is the key task from now on, he added.

"We will carry out the investigation objectively and scientifically," he said. "We will not miss a single point to get to the bottom of the cause of the accident, and restore the truth to prevent similar accident from reoccurring."

While decoding the black boxes, investigation teams have also conducted preliminary probe on the crash site, pattern of the debris and impact traces to determine the aircraft's flight path and attitude as well as the power of impact when it approached the ground. Also, the teams have analyzed flight data recorded by air traffic control's system to restore the flying process of MU5735, he added.

Information on the aircraft and its cargos as well as passengers onboard and their belongings have also been examined, Zhu said.

He added that according to international civil aviation agreements, a preliminary report on the accident will be completed within 30 days after the crash. After all the investigations are concluded, a full report will be made public.