Investigators in Russia say they cannot rule out the possibility that an attack downed a jet over the Black Sea, with the loss of all 92 passengers and crew.
An external “mechanical impact” could have caused Sunday’s disaster, a flight safety official said, while confirming there had been no explosion.
But he added that it was just one of many theories, including technical failure and human error.
The defence ministry airliner was bound for Syria at the time.
Russian media have published what they said was the pilots’ final conversation, reportedly transcribed from a flight recorder recovered from the wreckage.
It suggests that the Tu-154 jet’s wing flaps – panels which help lift an aircraft – were not moving together, causing the crew to lose control.
Analysis of the plane’s “black boxes” may not be completed until the end of January.
On board the plane were 64 members of the famed Alexandrov military music ensemble, still known as the Red Army Choir, who were due to perform for Russian forces in Syria.
The plane, which was 33 years old, crashed soon after take-off from Adler airport near the city of Sochi, where it had landed for refuelling, en route from Moscow.
It came down in good weather, close to the shore, with no survivors.
Nineteen bodies and more than 230 body fragments have been recovered in a massive search of the sea, along with 13 big fragments of the plane and about 2,000 smaller fragments.
“There was no explosion on board,” Lt-Gen Sergei Bainetov, the Russian air force’s head of flight safety, told reporters on Thursday.
“But this isn’t the only type of terrorist act… It could have been any type of mechanical impact, so we don’t rule out a terrorist act.”
Around seven theories that include poor quality fuel causing engine failure and a bird getting caught up in the engines are being investigated, he added, quoted by Reuters news agency.
“There is no leading theory,” he stressed.
However, Russian Transport Minister Maxim Sokolov told the same news conference: “The version of a terror attack isn’t being considered as the main version.”
Russian officials previously suggested terrorism was not being considered as a possible cause of the crash.
Last words attributed to the pilots:
- “Speed 300 [inaudible] .”
- “[Inaudible] .”
- “I’ve pulled in the landing gear, commander.”
- “[Inaudible] .”
- “Oh bloody hell!”
Piercing alarm sounds
- “The flaps, hell, what a…!”
- “The altimeter [altitude meter] !”
- “We’re in… [inaudible] .”
Alarm sounds about dangerous proximity to the ground
- “[Inaudible] .”
- “Commander, we’re falling!”
The Tupolev airliner was an old model no longer flown by airlines in Russia but still used by the military.
If the final conversation attributed to the crew proves to be genuine, it could indicate either a mechanical fault or a fault by the crew, such as mistakenly retracting the flaps instead of the undercarriage.
Tupolev-154: Russian workhorse
- The backbone of Soviet and Russian airlines for decades
- Three engines, narrow-bodied and medium range
- Designed in the mid-1960s, came into service in 1972 and was modernised in 1986 with new engines and equipment
- Has seen 39 fatal accidents, although few were due to technical problems. Many were as a result of difficult weather conditions and poor air traffic control. A few were lost in conflicts including in Lebanon, Georgia and Afghanistan
- Not used in Russia’s civil aviation since 2009, but is still used by the military. Only about 50 in service worldwide