Alexei Navalny, Russia’s prominent opposition leader and a vocal critic of President Vladimir Putin, has passed away in a prison located within the Arctic Circle, according to the prison service. Navalny was serving a 19-year jail term on charges widely regarded as politically motivated, making him the most significant opposition figure in Russia for the past decade.

Moved to an Arctic penal colony late last year, Navalny’s imprisonment in one of the toughest jails raised concerns about his well-being. The prison service in the Yamalo-Nenets district reported that Navalny “felt unwell” after a walk on Friday and “almost immediately lost consciousness.” Despite the efforts of an emergency medical team to resuscitate him, Navalny could not be revived. The cause of death is currently under investigation.

Navalny’s lawyer, Leonid Solovyov, refrained from commenting immediately, while his close aide Leonid Volkov expressed uncertainty about the reported cause of death, stating, “Russian authorities publish a confession that they killed Alexei Navalny in prison. We do not have any way to confirm it or to prove this isn’t true.”

The international community quickly acknowledged Navalny’s courage as Putin’s leading domestic adversary. France described his death as payment for resisting Russian “oppression,” and Norway’s foreign minister held Russian authorities accountable for the tragic outcome. President Putin’s spokesman, Dmitry Peskov, mentioned that Navalny’s death had been “reported to the president,” who was on a visit to the city of Chelyabinsk at the time of the announcement.