Officials said the 49-year-old was serving an eight-day sentence for alleged drug violations when he died on May 14 in a detention center.
According to police, the death was caused by heart failure.
Officials say an investigation has been launched.
Tatarstan police have been under increased scrutiny since March after a 52-year-old suspect died after police in Kazan allegedly beat and raped him with a bottle.
Several police officers have been arrested and charged in that case, and Tatarstan’s Interior Minister Asgat Safarov resigned.
The case has led to a series of revelations about more cases of alleged beating and torture of suspects by police in Tatarstan and other Russian regions.