The pilot of a Russian airliner failed to follow controllers' instructions just before his jet crashed, killing 88 people, one controller has said.
Irek Bikbov, who was guiding the Boeing 737-500 as it prepared to land, said it climbed instead of dropping altitude and veered left, instead of right.
The crash happened early on Sunday near the city of Perm, in the Urals.
The jet, flown by Aeroflot subsidiary Aeroflot Nord, caught fire as it came in to land on a flight from Moscow.
Lead investigator Alexander Bastrykin linked the crash to "technical failure and a fire in the right engine".
The plane's flight recorders have been found and will be analysed.
According to Mr Bikbov, the pilot "was acting oddly - he wasn't following instructions".
Mr Bikbov, quoted by Interfax news agency, said he had told the pilot that he was gaining altitude, to which the pilot replied: "confirmed, but we are descending".
When the plane reached 1,200m, Mr Bikbov said, he gave instructions for a third turnaround and "the crew accepted the instructions but failed to act upon them".
"I gave him the course to turn right and (he) veered to the left. I asked him: 'Are you all right?' He said: 'Affirmative'."
Mr Bikbov added: "It is possible that something was going on and he did not want to tell".
After handing over to another controller, Mr Bikbov said, he noticed that the plane was losing height rapidly and then the crash happened.
The plane came down on the outskirts of Perm, just a few hundred metres from residential buildings, but no-one was hurt on the ground.
No sign of terrorism
The Perm region is holding a day of mourning on Monday.
Russian President Vladimir Putin shakes the hand of Gen Gennady Troshev (February 2003)
Victim Gen Troshev (left) advised Vladimir Putin on Cossack affairs
Giving his preliminary opinion, Mr Bastrykin, head of the federal prosecutors' investigative committee, told Russian media there was "much evidence" for the engine fault theory.
Russia's Transport Minister, Igor Levitin, said he had no information to suggest the cause of the crash had been a terrorist attack, or that the plane had exploded in mid-air.
Relatives of some of the dead have arrived in Perm and are being looked after by the local authorities.
There were 82 passengers on board, including seven children, and six crew.
Those killed include Gen Gennady Troshev, a former commander of Russian forces in Chechnya, and 21 foreign citizens - nine people from Azerbaijan, five from Ukraine and one person each from France, Switzerland, Latvia, the United States, Germany, Turkey and Italy.
Contact with the plane was lost at 0521 Perm time on Sunday (2321 GMT Saturday).
The minister for security in the region said the plane had caught fire in the air at an altitude of 1,000m.
Part of the Trans-Siberian railway was shut down as a result of damage to the main east-west train track and the blaze took two hours to extinguish.
An eyewitness said the descent of the plane had looked like a "burning comet".
"I felt an explosion - I felt as it threw me up from the bed maybe half a metre up. Then my daughter ran in from the next room crying: 'Has a war begun or what?'" the unnamed woman told Russian TV.
"As the witness and neighbours are saying, it started burning still in the air.
"It looked like a comet, a burning comet. It hit the ground opposite the next house, there was a blaze, like fireworks, it lit the whole sky, the blaze."
Russian federal prosecutors have launched an inquiry to examine whether safety procedures were violated.
Aeroflot says the plane had "a full technical inspection" early this year and was judged to be in a "proper condition".
Sunday's accident was the deadliest involving a Russian airliner since 170 people died in August 2006 when a Tupolev-154 bound for St Petersburg crashed in Ukraine.
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