A Russian chemist who helped develop the Soviet-era nerve agent used to poison a former Russian double agent in southern England said only the Russian government could have carried out the attack with such a deadly and advanced toxin.

Vil Mirzayanov, 83, said he had no doubt that Russian President Vladimir Putin was responsible, given that Russia maintains tight control over its Novichok stockpile and that the agent is too complicated for a nonstate actor to have weaponized.

"The Kremlin all the time, like all criminals, denying — it doesn't mean anything," Mirzayanov said in an interview in his home in Princeton, New Jersey, where he has lived in exile for more than 20 years.