Add rain to the list of challenges facing Chinese investigators trying to determine why a passenger jet with 132 people on board crashed in a remote forested region this week.

The search for the black boxes and any survivors — a remote possibility at best — was temporarily suspended Wednesday due to rain on the muddy, charred mountainside.

The China Eastern Airlines Boeing 737-800 was flying at 29,000 feet (8,800 meters) on Monday afternoon when it suddenly nosedived into a gap in the mountains outside the southern Chinese city of Wuzhou.

WHAT HAVE RESCUERS FOUND?

Rescuers with sniffer dogs and drones have found wallets, identity cards and small parts of plane debris. Authorities have given no indication they found survivors, bodies or the plane’s “black box” flight recorders.

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Parts of the plane are scattered over a wide area, including the other side of the mountain, state broadcaster CCTV said. The main crash area, now a large barren pit in the forested mountainside, is about half the size of a football field.

Searchers must climb steep inclines as they fan out in the area, which is surrounded on three sides by mountains and accessible by a dirt road.

WHAT ARE INVESTIGATORS SAYING?

Investigators have declined to discuss possible reasons for the crash. Zhu Tao, director of the Office of Aviation Safety of the Civil Aviation Administration of China, said damage to the aircraft was severe, which makes the investigation “very difficult.”

“We cannot have a clear assessment of the cause of the accident with the information currently available,” Zhu said at a Tuesday night news conference, the first since the crash..

Authorities are “carrying out in-depth investigation” of the aircraft’s design and maintenance, air traffic control, weather and other issues, he said.

WHAT ABOUT THE BLACK BOXES?

Investigators will make an all-out effort to collect evidence, with a focus on finding the flight recorder, Tao said.

Recovering the so-called black boxes is key to the investigation – they are usually painted orange for visibility, but the longtime name has stuck. One device, called the flight data recorder, captures information about the plane’s airspeed, altitude, direction up or down, pilot actions, and performance of all key systems. The cockpit voice recorder captures sounds including conversations and background engine noise during the flight.

Even with the extent of the damage to the plane, investigators should be able to get a good idea of what happened if the black boxes survived and can be downloaded.

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WHAT IS KNOWN ABOUT THE PLANE?

The 6 1/2-year-old plane’s “technical condition was stable” and met requirements to fly, said Sun Shiying, the the chairman of the Yunnan provincial branch of China Eastern Airlines.

The flight had departed the city of Kunming in Yunnan and was headed to Guangzhou in Guangdong province.

The 737-800 has an excellent safety record, said Hassan Shahidi, president of the Flight Safety Foundation. It is from an earlier generation of the 737 series than the Boeing 737 Max jets, which were grounded after crashes in 2018 and 2019. The 737-800 does not have the flight control software that was blamed for the 737 Max crashes.

HOW HAS THE CHINESE GOVERNMENT RESPONDED?

The central government has sent a team led by Vice Premier Liu He and a senior Cabinet official, State Councilor Wang Yong, to the site to “guide rescue work” and the investigation of the crash