A possible cause for the Russian plane crash has emerged. Russia's civil aviation chief says the plane that crashed in Siberia, killing 31 people, may not have been de-iced properly before take-off. But he said there was no indication that negligence led to the crash.

It is not yet fully clear why the plane fell, but evidence has surfaced that it may not have been properly treated with de-icing agents. But Russian News & Information Agency has also said there is no basis so far "to connect this with the cause of the crash."

Investigators believe a technical fault in the French/Italian-made aircraft may be to blame. As witnesses reported seeing smoke coming from its engines as the plane came down and said its pilots had tried to return to the airport.

Rescuers and investigators work at the site of plane crash near Russia's Siberian city of Tyumen, on April 2, 2012. At least 31 people were killed when a passenger plane crashed in Russia's Siberia region early Monday, emergency authorities said. [Xinhua/RIA Novosti news agency]

The federal Investigative Committee say that while equipment failure appears to be the most likely cause of the crash, pilot error or mistakes by traffic controllers could not be ruled out.

Twelve people have been hospitalized and are in serious condition.

The crash is the worst in Russian history since a passenger plane slammed into a riverbank near the city of Yaroslavl after takeoff in 2011, killing 44 people and the entire Lokomotiv Yaroslavl ice hockey team.