European officials have announced Saturday that Russian opposition leader Alexei Navalny was killed with a rare toxin derived from South American poison dart frogs, challenging Moscow’s long-standing claim that he died of natural causes while imprisoned.

The findings, presented at the Munich Security Conference by representatives from the U.K., Sweden, the Netherlands, and Germany, confirm that Navalny’s death on February 16, 2024, was not natural. Forensic analysis identified epibatidine, a toxin up to 200 times more potent than morphine, in biological samples from Navalny’s body.

“Only the Russian government had the means, the motive, and the opportunity to deploy this lethal toxin against Alexei Navalny during his imprisonment,” said U.K. Foreign Secretary Yvette Cooper. She added that the use of this poison underscores the Kremlin’s approach to silencing political opposition.

Navalny’s widow, Yulia Navalnaya, who now leads the Russian opposition in exile, confirmed that she always suspected foul play. “We saw him just one day before — yes, by video connection — but he was laughing. He looked absolutely healthy,” she said. “It was obvious to me that something horrible happened to him. I was sure that it was Vladimir Putin who killed him.”

Navalnaya referenced a 2020 Novichok poisoning attempt against her husband in Russia. After recovering in Germany, Navalny returned to Russia, where he was immediately arrested and later sent to a high-security penal colony above the Arctic Circle, where he died.

The European governments formally notified the Organisation on the Prohibition of Chemical Weapons, alleging a breach of the Chemical Weapons Convention. French Foreign Minister Jean-Noël Barrot said the findings demonstrate Moscow’s willingness to use prohibited substances against its own citizens.

Thousands of mourners attended Navalny’s funeral in Moscow, and hundreds were detained for laying flowers in his honor. Western officials say the evidence now leaves little doubt about the Kremlin’s involvement.

The Kremlin has not publicly responded to the latest findings. Navalny’s widow stated that the scientific confirmation allows her to continue her husband’s anti-corruption and democratic work with clarity on the circumstances of his death.