The potent nerve agent linked to the poisoning of a Russian ex-spy in Britain may have been developed in the U.S. and the U.K. rather than Russia, a senior Russian has said.

Britain’s Foreign Secretary Boris Johnson said Friday that it was “overwhelmingly likely” that Russia’s President Vladimir Putin directed the March 4 poisoning of former military intelligence officer Sergei Skripal and his daughter in the English city of Salisbury. Britain has said that the weapon used in the attempted murder was believed to be from the Novichok family of nerve agents, which were reportedly first developed by the Soviet military.