The Antonov An-2 biplane was reported missing from an airfield in the town of Serov June 11, 2012, and it wasn't until May 2013 that hunters came across the wreckage and the victims' remains in a remote, mountainous area in the Urals.
Regional police said the plane was taken by pilot Khatib Kashapov and a group of friends while they were drinking.
Investigators said Tuesday the crash was caused by a mistake Kashapov made while intoxicated.
"He was flying at an inadmissibly low altitude; the plane caught a tree and fell down," the head of the regional investigation department on transport, Dmitry Putintsev, said. "According to preliminary reports, the pilot was in a state of alcoholic intoxication."
Kashapov's family has called for the criminal case against him to be dropped.
"They believe he's innocent. The first part of the criminal case will be forwarded to the court in February. The case is largely finished; we have experts' conclusions and the results of expert examinations," Putintsev said.