A Russian plane carrying 92 people to an air base in Syria crashed Sunday into the Black Sea minutes after taking off from the city of Sochi, Russia’s Defense Ministry said. Thousands of rescue workers were searching the undersea crash site but there appeared to be no survivors. Those on board included dozens of members of Russia’s world-famous army choir.

The cause of the crash wasn’t immediately known. At least one expert cited a terror attack as a possible reason — a scenario rejected by Russian officials.

In all, 84 passengers and eight crew members were on the Tu-154 plane when it disappeared from radar two minutes after taking off in good weather. Emergency crews found fragments of the plane about 1.5 kilometers (less than one mile) from shore and by Sunday afternoon, rescue teams had already recovered 10 bodies.

The plane belonged to the Defense Ministry and was taking the Alexandrov Ensemble to a New Year’s concert at Hemeimeem air base in Syria’s coastal province of Latakia. Those on board also included nine Russian journalists and a Russian doctor famous for her work in war zones. New Year’s is the main holiday for most Russians.

Russian President Vladimir Putin went on television to declare Monday a nationwide day of mourning.

“We will conduct a thorough investigation into the reasons and will do everything to support the victims’ families,” Putin said.

More than 3,000 people — including dozens of divers — worked from 27 ships and several helicopters to search the crash site, according to the Defense Ministry. Drones were also flown over to help spot bodies and debris. About 100 more divers were being flown in from naval facilities across Russia, and powerful spotlights were brought in so the search could continue around the clock.

Magomed Tolboyev, a decorated Russian test pilot, said the circumstances of the crash indicated that all on board had died.

“There is no chance to survive in such situation,” he said, according to the Interfax news agency. “The plane gets instantly blown into pieces.”

The Tu-154 is a Soviet-built three-engine airliner designed in the late 1960s. More than 1,000 have been built, and they have been used extensively by carriers in Russia and worldwide. The plane that crashed was built in 1983, and underwent factory check-ups and maintenance in 2014 and earlier this year, according to the Defense Ministry.

Viktor Ozerov, head of the defense affairs committee in the upper house of the Russian parliament, said the crash could have been caused by a technical malfunction or a crew error. He said it could not have been a terror attack because the plane was operated by the Russian military.

“I totally exclude” the idea of an attack bringing down the plane, he said according to the state-run RIA Novosti news agency.

However, some experts contested Ozerov’s claim, saying the crew’s failure to report a malfunction pointed at a possible terror attack.

“Possible malfunctions … certainly wouldn’t have prevented the crew from reporting them,” Vitaly Andreyev, a former senior Russian air traffic controller, told RIA Novosti, adding that it points at an “external impact.”

Russian planes have been brought down previously by terror attacks.

In October 2015, a Russian passenger plane carrying mostly Russian tourists back from vacation in Egypt was brought down by a bomb over the Sinai, killing all 224 people aboard. Officials said the explosive device was planted in the plane’s luggage compartment. The local affiliate of the Islamic State group claimed responsibility.

In August 2004, two Russian planes were blown up in the skies over Russia by suicide bombers, killing 89 people. A Chechen warlord claimed responsibility for the twin attacks, which happened on the same day.

The Russian military has repeatedly flown groups of Russian singers and artists to perform at Hemeimeem, which serves as the main hub for the Russian air campaign in Syria.

The passenger list for the Sochi plane released by the Defense Ministry included 64 members of the Alexandrov Ensemble, including its leader, Valery Khalilov. The ensemble, often referred to as the Red Army choir, is the official choir of the Russian military and also includes a band and a dance company. The choir sang “Get Lucky” at the opening of the 2014 Winter Olympic Games that Russia hosted in Sochi, becoming an instant online sensation.

The damage to the choir was reported to be extensive. Viktor Yeliseyev, head of the rival choir of the Russian National Guard, said “most singers of the choir have died.”

Also on board was Yelizaveta Glinka, a Russian doctor who has won wide acclaim for her charity work, which has included missions to war zones in eastern Ukraine and Syria. Her foundation said Glinka was accompanying a shipment of medicine for a hospital in Syria.

Putin presented Glinka with an award earlier this month.

“We never feel sure that we will come back alive,” she said at the Kremlin award ceremony. “But we are sure that kindness, compassion and charity are stronger than any weapon.”

Russian Prime Minister Dmitry Medvedev described the crash as a “terrible tragedy.” Defense Minister Sergei Shoigu was personally coordinating the rescue efforts.

In recent years, Russian airlines have replaced their Tu-154s with more modern planes, but the military and some other government agencies in Russia have continued to use them. While noisy and fuel-guzzling by modern standards, the plane has been popular with crews that appreciate its maneuverability and ruggedness.

“It’s an excellent plane, which has proven its reliability during decades of service,” veteran pilot Oleg Smirnov said in televised remarks.

In April 2010, a Tu-154 carrying Polish President Lech Kaczynski and 95 others crashed while trying to land at a sporadically used military airport in Smolensk in western Russia, killing everyone on board. Investigations by both Polish and Russian experts blamed pilot error in bad weather conditions, but Polish authorities have launched a new probe.

(AP)