European leaders said they are confident Navalny was poisoned with a toxin found in South American poison dart frogs.

Following accusations against Russia for poisoning Alexei Navalny, experts indicate the neurotoxin involved is likely a synthetic version.

The Kremlin has rejected the claim from several European nations and the widow of Russian opposition leader Alexei Navalny that Moscow silenced the critic using a poisonous substance found in South American dart frogs.

Russian opposition leader Alexei Navalny died in prison after being poisoned with a deadly toxin found in Ecuadorian dart frogs, the U.K. and other allies claimed Saturday

In a joint statement, the foreign ministries of the U.K., France, Germany, Sweden and the Netherlands say Navalny was poisoned by Russia with a lethal toxin derived from the skin of poison dart frogs.

There is no innocent explanation for the toxin, called epibatidine, being found in samples taken from Navalny's body, the Foreign Office said.

Mourners gathered in Moscow on Monday to mark two years since the death in custody of Russian opposition leader Alexei Navalny, under the shadow of a Kremlin crackdown and just two days since a new analysis reinforced suspicions that he was murdered.