Rescuers continued the search for survivors from Monday’s crash of a China Eastern Airlines aircraft in the hills of southern China, while the airline, China’s second-largest, grounded its entire Boeing 737-800 fleet.

As of March 22 evening, authorities said no survivors had been found from what is China’s biggest air tragedy in more than a decade.

More than 2,000 personnel from search teams were combing through the mountainous crash site as the focus turned to locating the aircraft’s black boxes and investigating the cause of the crash. Local authorities said both personnel and drones were being used to find the black boxes while the terrain in the mountains had posed logistical difficulties. The search site spanned 6,80,000 square metres, and drones will use thermal imaging to locate the black boxes.

The plane, with 132 people on board including nine crew, crashed a little over an hour after taking off on March 21 afternoon from Kunming, bound for Guangzhou.

Investigators will look to the black boxes to make sense of why the Boeing 737-800 suddenly dropped from cruising altitude. Flight tracking website FlightRadar24 said the plane was cruising at 29,100 feet before it suddenly and rapidly lost altitude, plunging 21,000 feet in a little over one minute and crashing into the hills near a village in southern Guangxi province. The last received data indicated a vertical speed of 31,000 feet per minute.

China Eastern Airlines said it had grounded its entire Boeing 737-800 fleet, although China’s two other major carriers, Air China and China Southern Airlines, did not do so. The 737-800 aircraft has had no known problems unlike the Boeing 737 Max, which Chinese regulators had grounded for more than two years after crashes of the new model in Indonesia and Ethiopia.

The last major crash in China was that of a Henan Airlines Embraer in 2010. That aircraft crashed while on approach in foggy weather and 44 people were killed.

The China Eastern crash is one of China’s worst ever air tragedies. The death in 1994 of 160 people after the crash of a Tupolev aircraft remains China’s biggest air disaster.

Over the past two decades, China has overhauled its aviation sector and established a robust safety record, phasing out older planes such as the Tupolev and operating a relatively young fleet of mostly Airbus and Boeing planes.