The main flight recorder from the Russian jet that crashed into the Black Sea on Sunday has revealed that faulty flaps were to blame, Russian media say.
The flaps, panels on the wings that help lift an aircraft, were not moving together, a source close to the probe told the private Interfax news agency.
The pro-Kremlin Life news website said this led the pilots to lose control as the plane was at a "critical angle".
It also quoted the crew's last words, including: "The flaps, hell... !"
The ageing Tu-154 airliner came down off the Russian coast with the loss of all 92 passengers and crew.
On board were 64 members of the famed Alexandrov military music ensemble, as well as one of Russia's best-known humanitarian figures, Yelizaveta Glinka.
The plane was heading to Russia's air force base in Syria where the choir was due to perform at a New Year's concert.
The latest findings allegedly come from a cockpit conversation stored on the flight's main "black box" data recorder, which was found underwater about a mile from the shore on Tuesday.
The military airliner was commanded by experienced pilot Maj Roman Volkov and his co-pilot was Capt Alexander Rovensky, who had 10 years of aviation service.
An earlier audio recording, played on Russian media, said to be of the final conversation between air traffic controllers and the plane's crew, revealed no sign of difficulties.
But Life, a new site which is close to the Russian security agencies, issued a transcript of the cockpit recording taken from the "black box", indicating the two pilots were taken by surprise.
The plane crashed soon after take-off from an airport near the city of Sochi, where it had landed for refuelling.
It disappeared from radar two minutes after taking off from Adler airport at 05:23 (02:23 GMT) on Sunday.
A second flight recorder has been found in a good condition and was raised from the seabed on Wednesday, the defence ministry said.
According to the authorities, so far 15 bodies have been recovered from the crash site.
The Tupolev airliner involved in the crash was an old model no longer flown by airlines in Russia but still used by the military. It was 33 years old.
The investigators have so far ruled out terrorism as a possibility, instead concentrating on human error, a technical fault or a combination of factors as being responsible.
Pilots' last words
- "Speed 300 (inaudible)."
- "I've pulled in the landing gear, commander."
- "Oh bloody hell!"
- "The flaps, hell, what a…!"
- "The altimeter [altitude meter]!"
- "We're in… (inaudible)."
- "Commander we're falling!"
Piercing alarm sounds
Alarm sounds about dangerous proximity to the ground