Alexei Navalny, Russia’s most prominent opposition leader and vocal critic of President Vladimir Putin, has died in state custody at the age of 47.

Navalny was serving a 19-year sentence at a penal colony in northern Russia known for its harsh conditions. According to a statement from Russia’s Federal Prison Service, Navalny was on a walk when he suddenly felt unwell and lost consciousness. Prison authorities attempted medical intervention but were unable to revive him.

The shocking news of Navalny’s premature death in custody has raised questions from his supporters, who note his history as a target of political persecution. Navalny survived a 2020 assassination attempt involving a military-grade nerve agent, which he blamed on Putin’s regime.

Upon returning to Russia in 2021, he was immediately imprisoned on charges he called politically motivated. His death closely resembles that of other Russian dissidents and opposition figures who died or were killed under suspicious circumstances.

Navalny rose to prominence through his extensive investigations into government corruption and by organizing large anti-Kremlin protests. He attempted to run against Putin in presidential elections but was barred from the ballot.

His death in state custody at age 47 marks a major blow to Russia’s political opposition and anti-corruption movement. Supporters say his imprisonment and now sudden death were attempts to silence a leading critic of Putin’s authoritarian rule.

An investigation into the exact circumstances surrounding Navalny’s death has been promised by Russian authorities. His family, supporters and human rights groups will be watching closely for answers.