Russian opposition leader Alexey Navalny was poisoned with a deadly toxin found in the skin of Ecuador dart frogs, the U.K.’s foreign office said on Saturday.

The toxin — epibatidine — was found in samples from Navalny’s body and “highly likely resulted in his death,” according to a statement issued by U.K. Foreign Secretary Yvette Cooper at the Munich Security Conference.

“Only the Russian state had the means, motive and opportunity to deploy this lethal toxin to target Navalny during his imprisonment in a Russian penal colony in Siberia, and we hold it responsible for his death,” the statement said.

Navalny, Russian President Vladimir Putin’s most prominent opponent, died in February 2024 at the age of 47 in a camp where he was serving a 19-year sentence for extremism. His supporters and Western governments blamed the Kremlin for his death.

The joint statement — made with Sweden, France, the Netherlands and Germany — said Russia had “brazenly” developed and deployed the poison in violation of the Chemical Weapons Convention.