A China Eastern Airlines Boeing 737-800 with 132 people on board plunged midflight and crashed in a mountainous area of southern China on Monday, according to officials and flight-tracking data.

Flight MU5735 left Kunming at 1:11 p.m. local time (1:11 a.m. ET) and was due to arrive at Guangzhou in the southeast of the country in less than two hours, according to information on FlightRadar24.

The plane was cruising at 29,100 feet and began a sharp descent after 2:20 p.m., recovering more than 1,000 feet briefly then resuming the dive before it lost contact. It fell more than 25,000 feet in about two minutes.

“This kind of tragedy is extremely unusual,” Richard Aboulafia, a managing director at AeroDynamic Advisory, said of the plane’s sudden, sharp drop from its cruising altitude.

China’s Civil Aviation Administration said it was sending a team to the crash site in the Guangxi region. Investigators will work to recover so-called black boxes that contain cockpit voice recordings and flight data. They are also likely to examine the aircraft’s previous flights, maintenance history, weather data and pilot health.

Chinese state media said the crash had caused a fire in the mountains. Video circulating on social media showed fire and smoke in the mountains and a section of a plane near a dirt path. Chinese officials have yet to confirm any passenger deaths.

If all 123 passengers and nine crew members are confirmed dead, it would be China’s deadliest airline crash since 1994 and the deadliest ever for China Eastern Airlines, according to the Aviation Safety Network database.

The 737-800 has one of the best safety records of any aircraft with 11 fatal accidents out of more than 7,000 planes that have been delivered to customers since 1997, according to aviation data and consulting firm Cirium.

The 737-800 that crashed Monday in China first flew in June 2015. It was not a Boeing 737 Max, the plane that was grounded worldwide after two fatal crashes in 2018 and 2019.

China Eastern Airlines confirmed the crash and the number of people on board via a statement on Weibo, which is China’s version of Twitter. The airline said it is sending workers to the site of the crash and has opened a hotline for family members.

The U.S. Federal Aviation Administration said it was aware of the crash and will assist in the investigation “if asked.”

Typically when a crash takes place, the country where the crash occurs leads the investigation. Because Flight 5735 was a U.S.-made plane, the National Transportation Safety Board has appointed an investigator for the crash. Boeing as the manufacturer and its regulator, the FAA, will likely assist.

“Our thoughts are with the passengers and crew of China Eastern Airlines Flight MU 5735,” Boeing said in a statement. “Boeing is in contact with the U.S. National Transportation Safety Board and our technical experts are prepared to assist with the investigation led by the Civil Aviation Administration of China.”

China Eastern, China’s second-largest carrier by capacity according to OAG, changed the colors of its website and social media accounts to black and white in a sign of mourning.

Chinese President Xi Jinping said he was “shocked” to learn of the incident, according to state media CCTV. He instructed Shanghai-based China Eastern to organize search and rescue efforts and begin an investigation into the cause of the crash.

The last fatal commercial passenger airline crash China was in 2010, when 42 people died on a Henan Airlines Embraer E-190 flight, according to the ASN database.

Boeing shares shed roughly 3.5% in Monday trading. Spirit AeroSystems, which makes 737 fuselages, likewise sold off 3.5%.

There are more than 4,200 737-800s in service worldwide and 1,177 in Chinese airlines’ fleets — the most of any country — according to Cirium.

Boeing has been trying to recover its reputation after the two crashes of the Max. The Boeing 737 Max was grounded worldwide in March 2019 after the second of the two crashes, occurring within five months of one another.

Indonesia’s Lion Air Flight 610 crashed on Oct. 29, 2018, and Ethiopian Airlines Flight 302 went down on March 10, 2019. All 346 people on board the two flights were killed in the crashes. 

China was the first country to ground the Max after the second crash. The U.S. and most other countries cleared the planes to return to service more than a year ago. Chinese regulators allowed the Max to resume flying in December, with changes, but the planes have not yet returned to service.

Boeing has been preparing to resume deliveries of its best-selling Max plane to China, a crucial milestone since airlines pay the bulk of an aircraft’s price upon delivery.

“If Boeing cannot deliver 737s to China, it will be that much harder to reduce the company’s 737 inventory, and this would likely make it more difficult to boost production from the [31 planes per month] planned for this year,” JP Morgan Boeing analyst Seth Seifman wrote in a note Monday.

Monday’s crash was a 737NG, or next generation, the model before the 737 Max.

Boeing reported its third annual loss in a row in January, disclosing $5.5 billion in costs tied to manufacturing flaws, which have forced the company to pause deliveries of the company’s 787 Dreamliners for most of the last 17 months.