Alexei Navalny, the most prominent critic of Russian President Vladimir Putin, known for his campaigns against official corruption and organizing large anti-Kremlin protests, passed away in prison on Friday (February 16), according to Russia’s prison agency. He was 47.

The Federal Prison Service released a statement stating that Navalny became unwell after a walk on Friday and subsequently lost consciousness. Despite efforts from an ambulance to revive him, Navalny did not survive. 

“The medical staff of the institution arrived immediately, and an ambulance team was called,” the prison service said.

“All necessary resuscitation measures were carried out, which did not yield positive results. Doctors of the ambulance stated the death of the convict…The causes of death are being established,” they added. 

Alexei Navalny was serving 19-year sentence

Navalny, serving a 19-year sentence on charges of extremism, was transferred in December from his previous prison in the Vladimir region of central Russia to a “special regime” penal colony, considered the highest security level of prisons in Russia, located above the Arctic Circle.

His relocation to the town of Kharp, situated in the Yamalo-Nenets region approximately 1,900 kilometers (1,200 miles) northeast of Moscow, was criticized by his allies as another attempt to suppress Navalny and stifle his voice.