The number of dead reported varied. The regional branch of the Investigative Committee said 37 people were killed, while an Emergencies Ministry representative said 15 bodies were recovered and 22 people were missing, RIA Novosti reported.
A criminal investigation has been opened into what caused the fire and documents seizures already have begun, Russia's Investigative Committee said in a statement posted on its website.
"According to preliminary information, the cause of the fire was carelessness on the part of one of the patients," the local branch of the Investigative Committee said.
The fire was contained to a building that served as a men's ward, RIA Novosti reported. The blaze was brought under control shortly after midnight Friday.
The Health Ministry said 59 people were in the single-story building when the fire started.
The Oksochi psycho-neurological facility cares for elderly people unable to look after themselves, the mentally ill and the chronically institutionalized, RIA Novosti said.
In April, a fire at a mental institution in Ramensky killed 38 people.
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