MOSCOW (Reuters) - Russia expanded its search on Monday for the remains of a military plane that crashed into the Black Sea, killing all 92 on board, and said pilot error or a technical fault - but not terrorism - were likely to have caused the tragedy.

The plane, a Russian Defence Ministry TU-154, was carrying dozens of Red Army Choir singers and dancers to Syria to entertain Russian troops in the run-up to the New Year.

Nine Russian journalists were also on board as well as military servicemen and Elizaveta Glinka, a prominent member of President Vladimir Putin’s advisory human rights council.

Divers and submersibles seeking the jet’s flight recorders scoured a stretch of water roughly 1 mile (1.6 km) from the southern Russian resort of Sochi.

Four small pieces of fuselage were recovered at a depth of 27 metres (89 ft), the RIA news agency said, but strong currents and deep water were complicating the search.

Major-General Igor Konashenkov, a Defence Ministry spokesman, said 11 bodies had been recovered. The ministry denied a RIA report that some of the dead passengers had been wearing life jackets.

He said the sea and air search operation, already involving around 3,500 people, was being expanded.

Putin designated Monday a nationwide day of mourning and flags flew at half-mast and TV stations removed entertainment shows from their schedules.

Prime Minister Dmitry Medvedev led a minute of silence at a government meeting, and mourners laid flowers at Sochi airport, from where the plane took off.

Kremlin spokesman Dmitry Peskov said military investigators were considering all theories, but that the version it may have been “a terrorist act” was “nowhere near the top of the list”.

FOUR THEORIES

The FSB security service said it had not so far found any evidence pointing to foul play and was investigating four possible causes, the Interfax news agency reported.

Those were that a foreign object had fallen into an engine, that the fuel had been poor quality causing engine failure, pilot error, or a technical fault.

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Mourners left flowers in front of the Moscow headquarters of the Russian Army’s Alexandrov song and dance troupe, more than 60 of whom were killed in the crash.

A handwritten note outside the office of Glinka, the late humanitarian worker on board, read: “We want V.V. Putin to ban the TU-154.”

The Defence Ministry says the downed jet, a Soviet-era plane built in 1983, had last been serviced in September and undergone more major repairs in December 2014.

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The last big TU-154 crash was in 2010 when a Polish jet carrying then-president Lech Kaczynski and much of Poland’s political elite went down in western Russia killing everyone on board.

Russian authorities have said they have no plans to withdraw the TU-154 from service for the time being.

Defence Ministry spokesman Konashenkov said 45 ships, five helicopters, drones, and more than 100 divers were involved in the wider search, and soldiers were scouring the Black Sea coastline as well.

He said ten bodies and 86 body fragments from the crash had been flown to Moscow so that experts could try to identify them.