A fire on Friday in a psychiatric hospital in Luka-Oksochi in northwestern Russia killed 37 people.

According to The New York Times, the fire, which started in the early morning, destroyed the one-story wooden hospital. Among the dead was a nurse who was trying to save patients.

A criminal investigation has been opened into the cause and reasons behind the fire, Russian news agency RIA Novosti reports. The Russian Investigative Committee stated that “According to preliminary information, the cause of the fire was carelessness on the part of one of the patients.”

The fire at the hospital, which may not have met current safety standards, is not the first such disaster to strike the country—fires in poorly maintained state run institutions are a persistent problem in Russia. The deadliest fire this year took place on April 2, also at a mental hospital, where 38 died and only 3 survived, and prompted President Vladimir Putin to examine Russia’s fire prevention standards and call for stricter attention to safety.