China said Wednesday one of two black boxes from the China Eastern plane crash was found in severely damaged condition. The recorder is so damaged that they are not able to tell whether it is the flight data recorder or the cockpit voice recorder.

Mao Yanfeng, the director of the accident investigation division of the Civil Aviation Authority of China, told a news conference that an all-out effort is being made to find the other black box.

Recovering the so-called black boxes is considered key to figuring out what caused the crash. The search for clues into why a Chinese commercial jetliner dove suddenly and crashed into a mountain in southern China had been suspended Wednesday as rain slickened the debris field and filled the red-dirt gash formed by the plane’s fiery impact.

Earlier, searchers had used hand tools, drones and sniffer dogs under rainy conditions to comb the heavily forested slopes for the flight data and cockpit voice recorders, as well as any human remains. Crews also worked to pump water from the pit created when the plane hit the ground, but their efforts were suspended around midmorning because small landslides were possible on the steep, slick slopes.

Video clips posted by China’s state media showed small pieces of the Boeing 737-800 plane scattered over the area. Mud-stained wallets, bank and identity cards have also been recovered. Each piece of debris has a number next to it, the larger ones marked off by police tape.

China Eastern Flight 5735 was carrying 123 passengers and nine crew from Kunming in Yunnan province to Guangzhou, an industrial center on China’s southeastern coast, when it crashed Monday afternoon outside the city of Wuzhou in the Guangxi region. All 132 people on board are presumed killed.

Investigators say it is too early to speculate on the cause. The plane went into an unexplained dive an hour after departure and stopped transmitting data 96 seconds into the fall.