Russian opposition leader Alexei Navalny died in prison Friday, the Russian prison service announced.

Navalny, 47, was one of Russian strongman Vladimir Putin’s most prominent critics, and had been serving his extended prison sentence in an Arctic penal colony. Russia’s Federal Prison Service said in a statement Friday Navalny passed away after feeling uneasy after a walk.

“On February 16, 2024, in penal colony No. 3, convict A.A. Navalny felt unwell after a walk, almost immediately losing consciousness,” the prison service for the Yamalo-Nenets region said on its website, according to NBC News.

“The facility’s medical workers immediately arrived at the scene and an emergency medical team was called in. All necessary resuscitation measures have been carried out, but they did not yield positive results. Emergency medics confirmed the death of the convict,” the statement added.

An attorney for Navalny did not immediately confirm the news of his death, a spokesperson of his said Friday on X, formerly known as Twitter.