Flight MU5735, operated by a Boeing 737-800 aircraft, crashed into a mountainside last Monday, killing all 132 people onboard in mainland China’s deadliest aviation disaster in 28 years.

“Large-scale air crash investigations that only rely on data provided by the black boxes are often insufficient to tell the full truth of the incident,” Zhu Tao, head of the aviation safety office at Civil Aviation Administration of China, told a press conference.

“While examining data from the black boxes, we’re doing our utmost effort to collect as much plane debris as possible and more footage and witness accounts of the accident,” said Zhu.

Recovery crews on Sunday found the second black box – the flight data recorder – from the wreckage of the jet.

So far, more than 15,000 rescue and search workers have been sent to the scene, covering a total of 370,000 square meters and more than 36,000 pieces of plane wreckage have been found, Zheng Xi, head of the Guangxi Fire and Rescue Corp, told the same briefing.

Earth-moving equipment has been brought in to conduct deep excavation work in the hope of recovering more plane parts and personal belongings of the passengers, Zheng added.