"The rescuers have so far not been able to find the third Tu-154 flight data recorder," the source said.
Divers have recovered two black boxes from the seabed and sent them to Moscow for decryption. the Russian Defense Ministry said earlier.
The aircraft heading from Moscow to Syria crashed shortly after refueling and take-off from the Russian southern city of Adler, near Sochi, on Sunday.
The Tu-154 was carrying 92 people, including eight crew members, 64 musicians of the Alexandrov Ensemble, nine reporters, a renowned charity worker, and two federal civil servants.
A law enforcement source said Thursday the main phase of the search and rescue operation was over after all main fragments and 19 bodies have been recovered from the Black Sea.
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