Notorious gang leader Sergei Tsapok, convicted of masterminding the murders of 12 people in 2010, died in prison on Sunday night.
Tsapok is the fourth member of his gang to die in prison recently, with the three previous deaths all deemed suicides, state news service RIA Novosti reported.
The most recent of these three deaths occurred on Friday, a hanging by a man sentenced to life in prison for murder.
Authorities suspect that Tsapok, whose gang was accused of terrorizing a rural southern Russian town into submission for over a decade, died of heart failure related to a stroke.
An investigation is under way to establish the exact cause of death, the Investigative Committee said Monday in a statement.
Tsapok, who is believed to have led the gang in the Krasnodar region town of Kushchyovskaya since 2002, was given a life sentence last year for his role in the murder of wealthy local farmer Server Ametov and 11 of his family members and friends.
The farmer had reportedly compromised Tsapok's power in the town by gaining popular support. The mass killing sent a shockwave throughout Russia.
Several members of Tsapok's gang were convicted of the murders, which took place on the night of Nov. 4, 2010, in the farmer's home. Tsapok was convicted of making the order.
An investigation found that Tsapok, who owned a private security firm and participated in local politics, had ruthlessly tormented local competitors into giving up control of the region's property and business assets.
On Saturday, Tsapok was taken to a hospital where he was preliminarily diagnosed with a stroke before returning to his prison's medical facilities for further treatment, the Investigative Committee said. His lifeless body was recovered from his prison cell on Monday morning.