Russia poisoned opposition politician Alexei Navalny with a toxin from a poison dart frog, France, Germany, the Netherlands, Sweden and the United Kingdom said on Saturday.

The five governments said their analyses of samples from Navalny’s body “conclusively confirmed” the presence of a toxin called epibatidine, which is found in poison dart frogs in South America and is not found naturally in Russia. “Navalny died while held in prison, meaning Russia had the means, motive and opportunity to administer this poison to him,” they said.

“There is no innocent explanation for its presence in Navalny’s body,” the U.K. said in a statement on Saturday. 

Navalny died in a Russian penal colony in February 2024. The Russian authorities said he died of natural causes.

But poisoning was “highly likely the cause of his death,” the five countries said in a joint statement on Saturday. Sky News reported that British scientists from Porton Down, the U.K.’s top secret laboratory, played a key role in the findings.

Navalny, a longtime thorn in the side of Russian President Vladimir Putin, was filmed the day before his death at a court hearing cracking jokes and seemingly in high spirits.

Yulia Navalnaya, his widow, announced the finding at a press conference Saturday at the Munich Security Conference, alongside foreign ministers from the European countries.

The U.K., in a separate statement, said that the circumstances of Navalny’s death were “brutal and barbaric.” 

The five countries’ representatives to the Organization for the Prohibition of Chemical Weapons have written to its Director General to inform him of the alleged Russian breach of the Chemical Weapons Convention, the countries said. 

The U.K. said Navalny’s death is part of an “alarming pattern” of behavior by Russia, including its use of Novichok in the U.K. in 2018, which led to the death of a British woman, and Russian troops’ “frequent use of chemical weapons on the battlefield in Ukraine.”

Russia’s foreign ministry did not immediately respond to a request for comment.