Rescue workers examine the wreckage of an ATR-72 plane near Tyumen in Siberia (AP)
A Russian airliner that crashed shortly after take-off killing 31 on board had not been properly de-iced, the country's civil aviation head said tonight.
However Alexander Neradko added there was no indication that the negligence caused the crash.
The ATR-72 turboprop operated by UTair took off from Tyumen in snowy Siberia and crashed two miles away. Twelve people survived but are in a serious condition in hospital. All of the 39 passengers and four crew on board were Russian.
Mr Neradko said the de-icing treatment "was not done at the necessary level" but investigators could not yet connect this with the causes of the crash.
Other areas being studied are a technical failure of the French/Italian-made twin-engine aircraft. Witnesses reported seeing smoke coming from its engines as the plane came down and said the pilots had tried to return to the airport.
The federal Investigative Committee said while equipment failure appeared to be the most likely cause of the crash, pilot error or other causes had not been ruled out.
Russia has seen a string of deadly crashes in recent years. Some have been blamed on the use of ageing Soviet-era aircraft, but industry experts point to a number of other problems, including poor crew training, crumbling airports, lax government controls and widespread neglect of safety in the pursuit of profits.
Pilot error was blamed for a September crash in Yaroslavl that killed 44 people, including a professional hockey team.
Pilot error and fog also were ruled the main causes of a crash in April 2010 that killed Poland's president and 95 other people as their plane was trying to land near Smolensk, in western Russia.
The ATR-72 has been involved in several accidents in past years. Most recently, one went down in bad weather in Cuba in November 2010, killing all 68 people on board. Cuban aviation officials said the investigation showed there was nothing wrong with the aircraft.