Chinese recovery crews have found one of the black box flight recorders from the China Eastern Airlines jet that crashed on Monday with 132 people on board, state media has reported.
The domestic passenger plane plunged from more than 20,000ft into a mountainous area of Guangxi, sparking an intense bamboo fire and almost disintegrating on impact.
Response officials said the circumstances of the crash meant investigators faced "a very high level of difficulty" in establishing a cause.
On Wednesday afternoon China's official state news agency, Xinhua, reported that one of two onboard flight recorders, which capture critical flight data and cockpit audio, had been recovered.
However, it had been "badly damaged", reports said.
"This is a major breakthrough in our research work," an official spokesperson told reporters.
The flight recorders – one that captures voice and another that captures flight data – are painted bright orange and built to withstand sudden impact, officials said on Wednesday.
"We know that before the crash there was a sudden dive of the plane within a very short period of time," a spokesperson said, adding that they were working to determine which one they had found.
Zhu Tao, director of aviation safety at the Civil Aviation Administration of China (CAAC), said on Tuesday no survivors had yet been found by rescue workers who continue to search the mountainous, forested site where the Boeing 737-800 came down.
"The jet was seriously damaged during the crash, and investigations will face a very high level of difficulty," Zhu said at the first government briefing on the disaster.
"Given the information currently available, we still do not have a clear assessment of the cause for the crash," he said, adding that the aircraft did not respond to repeated calls from air controllers during its rapid descent.
The jet "met airworthiness standards before taking off and technical conditions were stable," Sun Shiying, a China Eastern Airlines official, said at a briefing.
"The crew members were in good health, and their flying experience was in line with regulatory requirements."
Flight MU5735 was headed on Monday for the port city of Guangzhou from Kunming, the capital of the south-western province of Yunnan, when it plunged from cruising altitude to crash in the mountains of Guangxi less than an hour before its scheduled landing time.
The jet appeared to dive to the ground at an angle of about 35 degrees from the vertical in video images from a vehicle’s dashboard camera, according to Chinese media.
Rain in southern China on Wednesday had earlier halted the search for victims with risks of landslides that could endanger rescue workers scouring the difficult, heavily forested terrain, state television reported. The wet weather was forecast to last for the rest of the week.
Si, 64, a villager near the crash site who declined to give his first name, told Reuters he heard a "bang, bang" at the time of the crash. "It was like thunder," he said.
State television has shown images of plane debris strewn among trees charred by fire. Burnt remains of identity cards and wallets were also seen.
Police set up a checkpoint at Lu village, on the approach to the site, and barred journalists from entering. Several people gathered for a small Buddhist ceremony nearby to pray for the victims.
The last commercial jetliner to crash in China was in 2010, when an Embraer E-190 regional jet flown by Henan Airlines went down, killing 44 of the 96 aboard.
Highlighting the top-level concern, vice premier Liu He went to Guangxi on Monday night to oversee search and rescue operations.
An official of the same rank was similarly sent to the site of the 2010 crash in north-east China.
The disaster comes as Boeing seeks to rebound from several crises, notably the impact of the coronavirus pandemic on air travel and safety concerns over its 737 Max model after two deadly crashes.
"Accidents that start at cruise altitude are usually caused by weather, deliberate sabotage, or pilot error," Dan Elwell, a former head of US regulator the Federal Aviation Administration (FAA), told Reuters.
Elwell, who led the FAA during the 737-Max crisis, said mechanical failures in modern commercial jets were rare at cruise altitude.
The 737-800 was delivered on 22nd June 2015, and accumulated 18,239 hours of flight time after 8,986 flights, said Zhu at the CAAC.
Chinese investigators are leading the inquiry because the crash took place there, but US authorities will provide aid because the plane was US-made.