But poisoning was “highly likely the cause of his death,” the five countries said in a joint statement on Saturday. Scientists from Porton Down, the U.K.’s top secret laboratory, played a key role in the findings, according to British officials.
Navalny, a longtime thorn in the side of Russian President Vladimir Putin, was filmed the day before his death at a court hearing cracking jokes and seemingly in high spirits
The U.K. said in a separate statement on Saturday that the circumstances of Navalny’s death were “brutal and barbaric.”
The five countries’ representatives to the Organization for the Prohibition of Chemical Weapons have written to its director general to inform him of the alleged Russian breach of the Chemical Weapons Convention.
The U.K. said Navalny’s death is part of an “alarming pattern” of behavior by Russia, including its use of the Novichok chemical agent in the U.K. in 2018, which led to the death of a British woman, and Russian troops’ “frequent use of chemical weapons on the battlefield in Ukraine.”
Russia’s foreign ministry did not immediately respond to a request for comment.
This article has been updated.