Russia accused of killing Navalny with dart frog toxin. What we know about the rare poison

European allies' findings that Putin critic Alexei Navalny was killed with a rare toxin from poison dart frogs is "troubling," said Secretary of State Marco Rubio on Sunday, adding the U.S. is "not disputing" the report. The big picture: Rubio told reporters in Bratislava during a visit to Slovakia "we don't have any reason to question" the report that the U.K., France, Germany, Sweden and the Netherlands issued Saturday, which the Kremlin disputes. The report follows Yulia Navalnaya, the Russian opposition leader's widow, saying in September that that labs in "at least two countries" separately examined "smuggled" samples of Navalny's biological material and concluded he had been poisoned. What they found Lab tests concluded a "deadly toxin found in the skin of Ecuador dart frogs," epibatidine, was detected in samples from Navalny's body, per a Saturday British government statement, titled: "UK confirms Russia poisoned Navalny in prison with rare toxin." This "highly likely resulted" in the 47-year-old pro-democracy activist's 2024 death in a Siberian penal colony, according to the statement. What they're saying Russian Foreign Ministry spokesperson Maria Zakharova told state media the findings were a "Western propaganda hoax," adding: "When the test results are available and the formulas for the substances are disclosed, we will comment accordingly." Yes, but: A joint Saturday statement by the U.K., France, Germany, Sweden and the Netherlands noted that Russia had 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," per the statement. "Navalny died while held in prison, meaning Russia had the means, motive and opportunity to administer this poison to him." The statement noted a European probe found Navlny was left in a coma in a hospital in Berlin, Germany, in 2020 after being attacked with Novichok — the same never agent used in the 2018 attempted assassination of Sergei Skripal, a Russian former double agent who relocated to the U.K. A strawberry poison dart frog (Dendrobates pumilio). Photo: DeAgostini/Getty Images What is epibatidine? This highly toxic, natural alkaloid compound is found in the skin of the Ecuadorian poison frog, per a report published on the National Institutes of Health website. This toxin can be made in a laboratory, which European officials believe was the case in Navalny's death. "Dart frogs in captivity do not produce this toxin and it is not found naturally in Russia. There is no innocent explanation for its presence in Navalny's body," according to the British government statement. "Russia has brazenly developed and deployed this poison in violation of the Chemical Weapons Convention." How it works Epibatidine is 100 to 200 times more potent than morphine, according to the NIH. Its effects on humans is similar to those of Novichok: If entered into the bloodstream, it can cause "convulsions, paralysis and eventually death," notes Britain's University of Bristol in an online post Alastair Hay, emeritus professor of environmental toxicology at the University of Leeds, told Britain's Press Association the toxin blocks breathing "and any person poisoned dies from suffocation." The bottom line: The presence of epibatidine in human blood "suggests deliberate administration," Hay told PA. "If epibatidine ... was indeed used to poison Alexei Navalny, this is in violation of the 1972 Biological and Toxin Weapons Convention (BTWC) and the 1993 Chemical Weapons Convention (CWC)," he said. "The Soviet Union was a co-sponsor of the BTWC. Russia is a signatory of both the BTWC and CWC. If Russia used epibatidine to poison Mr Navalny it has violated two treaties it has sworn to uphold." More from Axios: How Navalny became Putin's biggest political foe Biden: "No doubts" Putin is behind Navalny's death Alexei Navalny said he knew he would die in prison in new memoir