Jailed Russian opposition leader and outspoken Kremlin critic Alexey Navalny, who made global headlines when he was poisoned with a nerve agent in 2020, has died aged 47, the Russian prison service said.

Navalny had long been a thorn in the side of President Vladimir Putin, exposing corruption in high places, campaigning against the ruling United Russia party, and orchestrating some of the biggest anti-government protests in recent years.

He returned to Russia in 2021 from Germany, where he had been treated after being poisoned with Novichok, a Soviet-era nerve agent.

Upon his return, Navalnvy was swiftly arrested on charges he dismissed as politically motivated. He has been incarcerated ever since, with longstanding concerns for his welfare growing more intense after he was transferred to a penal colony north of the Arctic Circle.

Navalny “felt unwell after a walk” and “almost immediately” lost consciousness, the prison service said Friday. It said it was investigating his “sudden death.”

Kremlin spokesman Dmitry Peskov said Putin has been informed of the reports and that it is for doctors to determine the cause of Navalny’s death.

“As far as we’re aware, by all the rules, the prison service is carrying out checks and clarifications,” Peskov told reporters. Asked about reports that the death involved a blood clot, he replied: “I don’t know. Doctors must find it out.”

Navalny’s spokesperson, Kira Yarmysh, said his lawyer is traveling to Kharp, where Navalny has been held since December.

“As soon as we have any information, we will report it,” Yarmysh wrote on X.

Navalany’s reported death comes shortly before Russia’s presidential election, set to take place on March 17, where Putin is widely tipped to win a fifth term in a move that would see him retain power until at least 2030.

Navalny was sentenced to 19 years in prison in August after he was found guilty of creating an extremist community, financing extremist activists, and various other crimes. He was already serving sentences of 11-and-a-half years in a maximum security facility on fraud and other charges he denies.

Four months later, his lawyers said they had lost contact with Navalny, who was believed to be imprisoned in a penal colony about 150 miles east of Moscow. He failed to appear at several scheduled court hearings in December. His legal team said on December 22 that he’d been missing for 17 days.

After filing 680 requests to locate Navalny, his team announced on December 25 that they had “found” him more than a thousand miles away at the IK-3 penal colony in Kharp, known as “Polar Wolf.”

“The conditions there are harsh, with a special regime in the permafrost zone. It is tough to get there, and there are no letter delivery systems,” said Ivan Zhdanov, director of Navalny’s anti-corruption foundation.