WUZHOU, China (AP) — Rough terrain and rainfall were hampering the search Wednesday for clues into why a China Eastern plane inexplicably fell from the sky and crashed into a wooded mountainside earlier this week, presumably killing all 132 people on board.

Under rainy conditions, searchers using hand tools, drones and sniffer dogs were combing the crash site and a debris field spread across steep, heavily forested slopes in southern China for the black boxes containing the flight data and cockpit voice recorders, a well as any human remains.

Video clips posted by China’s state media showed small pieces of the Boeing 737-800 plane scattered over the area, some in green fields, others in burnt-out patches with raw earth exposed. 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.

Investigators say it is too early to speculate on the cause of the crash. Flight 5735 went into an unexplained dive an hour after departure and the plane stopped transmitting data 96 seconds into the fall.

It crashed Monday afternoon outside the city of Wuzhou in the Guangxi region. The plane had been flying from Kunming, the capital of Yunnan province, to Guangzhou, an industrial center not far from Hong Kong on China’s southeastern coast.

An air-traffic controller tried to contact the pilots several times after seeing the plane’s altitude drop sharply, but got no reply, a grim-faced Zhu Tao, director of the Office of Aviation Safety at the Civil Aviation Authority of China, said at a Tuesday evening news conference.

“As of now, the rescue has yet to find survivors,” Zhu said. “The public security department has taken control of the site.”

Kang reported from Kunming, China. Associated Press researcher Yu Bing and news assistant Caroline Chen in Beijing, researcher Si Chen in Shanghai, video producer Olivia Zhang in Wuzhou, China, writer Adam Schreck in Bangkok and airlines writer David Koenig in Dallas contributed to this report.