Russia’s prison agency announced Friday that prominent Vladimir Putin critic and opposition leader Alexei Navalny has died at the age of 47.

The Federal Prison Service said in a statement that Navalny felt unwell after a walk on Friday and lost consciousness, according to The Associated Press. An ambulance arrived to try to rehabilitate him, but he died, the statement added.

Navalny’s spokesperson said in a post on X that “we have no confirmation of this yet.

“Alexei’s lawyer is currently on his way to Kharp. As soon as we have some information, we will report on it,” Kira Yarmysh added.

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Navalny was being held at the IK-3 penal colony, also known as “Polar Wolf,” in Kharp in northern Russia, which is considered one of the country’s toughest prisons.

Previously, Russian authorities had held him at a facility roughly 145 miles east of Moscow. Navalny’s team lost contact with him after he failed to appear in court via video link for a hearing on Dec. 5, kicking off a desperate search until he resurfaced in Kharp around Christmas.

In 2021, President Biden, after meeting with Putin in Geneva, Switzerland, was asked about what would happen if Navalny died in Russian custody.

“I made it clear to him that I believe the consequences of that would be devastating for Russia,” Biden said.

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Navalny has previously organized anti-government demonstrations and has run for office to advocate for reforms against what he claims is corruption in Russia. He was the victim of an alleged assassination attempt in 2020, when he suffered poisoning from a suspected Novichok nerve agent. 

He remained in a coma for several weeks while doctors in Germany fought to keep him alive. He accused Putin of being responsible for his poisoning.

Navalny then returned to Russia in 2021, when authorities immediately arrested him and later sentenced him to 19 years in prison on extremism charges. His team has repeatedly raised concerns about his treatment following his return and Navalny has said the charges were politically motivated.

The remote region where Navalny was being held is notorious for long and severe winters. Kharp is about 60 miles from Vorkuta, whose coal mines were part of the Soviet gulag prison-camp system.

Whenever Putin spoke about Navalny, he made it a point to never mention the activist by name, referring to him as “that person” or similar wording, in an apparent effort to diminish his importance, according to The Associated Press.

Last August, Yevgeny Prigozhin, the founder of Russia’s Wagner Group who challenged the rule of Putin, was killed in a plane crash outside of Moscow. The White House then appeared to formally acknowledge that the government believes Prigozhin was assassinated by Putin. 

Fox News’ Michael Dorgan, Chris Pandolfo and The Associated Press contributed to this report.