Chairman Alexey Motozov of the CIS Interstate Aviation Committee said one of the inexperienced pilots flying the plane inadvertently applied the brakes while taking off, the Russian news agency RIA Novosti reported.
Motozov told a news conference the action slowed the plane's speed to a level that was "less than that required for the actual take-off weight of 54 tons and nominal power regime of the engines."
In addition, neither pilot was fit for flying, he said.
One pilot had traces of a sedative in his blood and the other had been diagnosed with a disease that affected motor skills.
The Yak-42, chartered by a Russian hockey team, was headed to Minsk when it cashed in the Yaroslavl region some 160 miles northeast of Moscow. Among those killed were former NHL defenseman Brad McCrimmon and forward Pavol Demitra.