The United Kingdom has alleged that Russian opposition leader Alexei Navalny was killed by poisoning, based on findings presented at a security conference in Berlin on Saturday. British representatives said they and their allies verified samples from Navalny’s body, concluding that a toxic substance was used. UK Foreign Secretary Yvette Cooper stated that only the Russian government had the means to carry out such poisoning.

Navalny died two years ago while imprisoned in a Siberian facility. According to Russia’s state news agency TASS, Moscow dismissed the British claim as propaganda. Cooper said tests revealed the presence of epibatidine, a poison naturally found in certain South American frogs, and argued that it could not occur naturally in Russia or be produced by prisoners or domestic animals.

Cooper added that Russia viewed Navalny as a threat and that the use of such a rare toxin demonstrated the government’s fear of political opposition.