Russian opposition leader Alexey Navalny in court in Moscow in 2019. 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 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