"During search operations, one of the black boxes was found under the aircraft`s cabin. It will be taken out of the water soon," the source said.

The condition of the black box is yet unknown, the source added. "Two other two boxes were in the tail section of the aircraft. They have not been found yet," the source said.

Possible causes of the tragedy

One of the priority causes of the Tu-154 plane crash now is the penetration of foreign objects in the engine, the source said:

"One of the key versions of the crash at the moment is the penetration of foreign objects in the engine," the source said, adding that other possible causes such as pilot error and technical malfunction were also being considered.

Russia’s Federal Security Service said on Monday that investigators were considering four main possible causes of the crash - penetration of foreign objects in the engine, substandard fuel that caused the loss of thrust, pilot error and technical malfunction of the plane.

According to the FSB’s latest conclusions, no evidence has been found linking the crash to terrorism.

A Tu-154 plane from Russia’s Defense Ministry crashed in the early morning hours of December 25 shortly after taking off from the Black Sea resort of Sochi.

There were 92 people on board the aircraft, including eight crew members and 84 passengers that lost their lives in the plane disaster. Among those on the fatal flight was the Executive Director of the Spravedlivaya Pomoshch (Fair Aid) charity fund, Elizaveta Glinka, better known to the Russian public as Dr. Liza, as well as military servicemen and nine reporters from Russia`s Channel One, Zvezda and NTV networks. The plane was also carrying members of the world-renowned Alexandrov Ensemble, an official army choir of the Russian Armed Forces. The ensemble was on its way to celebrate New Year’s Eve with Russia’s Aerospace Forces at the Hmeymim air base in Syria.

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