European governments said Thursday they have concluded that Russian opposition leader Alexei Navalny was poisoned with a rare and highly toxic substance known as epibatidine, a compound derived from South American poison dart frogs.

In a joint statement, the United Kingdom, France, Sweden, the Netherlands and Germany said laboratory analyses of biological samples confirmed the presence of the lethal toxin. Epibatidine is an extremely potent neurotoxin that can cause respiratory failure.

Navalny, 47, died in February 2024 while imprisoned in an Arctic penal colony, according to Russia’s Federal Penitentiary Service. Russian authorities said at the time that he died of natural causes.

“Russia claimed that Navalny died of natural causes. But given the toxicity of epibatidine and reported symptoms, poisoning was highly likely the cause of his death,” the European governments said. They added that because Navalny was in state custody, Russia had “the means, motive and opportunity” to administer the substance.

Navalny’s widow, Yulia Navalnaya, said in a social media post that the findings confirmed her long-held belief that her husband was poisoned.

“I was certain from the first day that my husband had been poisoned, but now there is proof: Putin killed Alexei with chemical weapon,” she wrote, thanking the countries involved in the investigation. “Vladimir Putin is a murderer. He must be held accountable for all his crimes.”

Russian officials did not immediately respond to the allegations.

Navalny, a prominent critic of President Vladimir Putin, had been sentenced in 2023 to 19 years in prison on extremism charges, in addition to a nearly 12-year sentence he was already serving. Later that year, he was transferred to a “special regime” facility north of the Arctic Circle.