Failure to de-ice a UTair passenger plane before takeoff was the most likely cause of a crash in Siberia that killed 31 people, federal investigators said on Wednesday.
The UTair ATR 72-200 aircraft, carrying 43 people, tilted to its right side and then sharply left as it crashed shortly after takeoff from Tyumen in western Siberia on Monday.
"At the moment, on the basis of the materials received, the investigators consider the failure to conduct a de-icing procedure to be the most probable cause of the air crash," Russia's Investigative Committee said on its website.
It said witness statements from those responsible for de-icing the plane and video evidence from the airport's surveillance systems supported the suspicion. UTair officials declined immediate comment.
The head of Russia's federal air transport agency Rosaviatsiya, Alexander Neradko, had also said earlier that the plane had not been de-iced properly.
UTair has suspended flights of ATR 72-200s pending results of a separate investigation by the Interstate Aviation Committee, which looks into air crashes in 12 former Soviet republics.
UTair director Andrei Martirosov said, however, that the decision was made to reassure passengers and that there were no grounds for doubt so far that the plane was in a good technical condition.
UTair said the plane crashed while trying to make an emergency landing.
The federal investigators said the plane had notched up 35,000 flying hours since going into operation in 1992 and had not had a "serious" technical check since 2010.