Russia's intelligence agency FSB said Monday it sees no signs of a possible terror plot in Sunday's plane crash over the Black Sea.
All 92 people on the Russian military plane are believed to have died when it crashed two minutes after taking off from the southern city of Sochi. The passengers included dozens of singers in Russia's world-famous military choir, nine Russian journalists and a Russian doctor known for her charity work in war zones.
The FSB said Monday in a statement quoted by Russian news agencies that it "has not found any signs or facts pointing to a possible terror attack or sabotage on board."
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The intelligence agency says it is focusing the probe on possibilities including pilot error, low quality of fuel, external objects getting in the engine or an unspecified technical fault.
On Monday, the search operation involved more than 3,500 people on 45 ships — including 135 divers flown in from across Russia — sweeping a vast crash site at sea and along the shore, according to the Defense Ministry.
Five helicopters and drones were being used to help spot bodies and debris. Two deep-water submersibles arrived Monday in Sochi to aid the operation, which went on all through the night thanks to powerful searchlights.
The Black Sea search area — which covers over 10 square kilometers (about 4 square miles) — is plagued by underwater currents that can carry debris and body fragments into the open sea.
The search party has not yet found the plane's black boxes, Emergency Situations Minister Vladimir Puchkov told Russian news agencies. Transport Minister Maxim Sokolov said earlier that the plane's flight recorders did not have radio beacons, so locating them on the seabed was going to be challenging.
Still, several aviation experts noted factors that could suggest a terror attack, such as the crew's failure to report any malfunction and the fact that debris from the plane was scattered over a wide area.
"Possible malfunctions ... certainly wouldn't have prevented the crew from reporting them," Vitaly Andreyev, a former senior Russian air traffic controller, told RIA Novosti.
The plane was taking the Defense Ministry's choir, the Alexandrov Ensemble, to perform at a New Year's concert at Hemeimeem air base in Syria's coastal province of Latakia. It had originated from Moscow's military airport of Chkalovsky and stopped in Sochi for refueling.
Despite the Syrian connection, Sokolov said the government sees no need to heighten security measures at Russian airports.
Emergency crews found fragments of the plane about 1.5 kilometers (1 mile) from the shore Sunday but a deputy defense minister told Russian news agencies that experts estimated the Tu-154 crash site at 6 kilometers (3.7 miles) from the shore.
By Monday morning, rescue teams had recovered 11 bodies as well as body fragments. All were flown to Moscow, where the remains will be identified. Local governor Veniamin Kondratyev told Russian state television that most of the bodies could be still inside the fuselage.
Russia also asked the authorities of Georgia's breakaway republic of Abkhazia, which lies just 7 kilometers (4.3 miles) east of the Sochi airport, to help monitor the Black Sea for plane debris or bodies.
The Tu-154 is a Soviet-built three-engine airliner designed in the late 1960s. The plane that crashed Sunday was built in 1983, and underwent factory check-ups and maintenance in 2014, the Defense Ministry said.
On Monday, a nationwide day of mourning for the plane victims, red and white carnations piled up outside the Moscow office of the Alexandrov Ensemble. One singer who did not get on the plane for personal reasons said he was devastated at the loss of so many talented colleagues.
Soloist Vadim Ananyev stayed behind to help his wife with the kids as they just had a new baby.
"I have lost my friends and colleagues, all killed, all five soloists - I feel in complete disarray," Ananyev told The Associated Press. "It is such a shame. I have known these people for 30 years. I know their wives and children. I feel terrible for the children and for all that I have lost."
Ananyev said he had received condolences from all over Russia and from abroad.
"We were loved all over the world, never mind the political situation," he said.
Mourners also lit candles and brought flowers to Channel One and NTV, whose TV journalists were going to Syria to cover the concert, and to a charity founded by Dr. Yelizaveta Glinka, who was on the plane bringing medicines to Syria.
At Sochi's Adler airport, mourners came to light candles at the airport's chapel and lay flowers at an improvised shrine that featured photos of some of the victims.
Russian TV channels took entertainment shows off their programs Monday and outdoor seasonal celebrations were scrapped across Russia.