The British Police on Wednesday said the substance that rendered a former Russian double agent critically ill in Salisbury city was a nerve agent, BBC reported. Sergei Skripal and his daughter Yulia were found unconscious on Sunday on a bench outside the Maltings shopping centre. Both of them are said to be in serious condition in hospital.

“This is being treated as a major incident involving an attempted murder by the administration of a nerve agent,” London Assistant Commissioner Mark Rowley said, according to Reuters. “I can also confirm that we believe the two people originally who became unwell were targeted specifically.”

Rowley, however, refused to identify the specific nerve agent used in the attack. There was no evidence of a widespread health risk to the public, he added.

In 2006, Russia convicted the 66-year-old for treason after he gave up the names of a dozen Russian spies working in Europe to the United Kingdom’s Secret Intelligence Service, MI6. A former colonel in Russia’s GRU military intelligence service, Skripal was given refuge in Britain after he was exchanged for Russian spies caught in the West, in a Cold War-style “spy swap” at the Vienna airport.

On Tuesday, United Kingdom Foreign Secretary Boris Johnson said his country would respond “robustly” if it found that Russia was behind Skripal’s condition. Moscow denied involvement and criticised Johnson for his “wild” comments. “It’s very hard not to assess this [speculation] as provocative black PR designed to complicate relations between our two countries,” Foreign Ministry spokeswoman Maria Zakharova said on Wednesday.

Toxic nerve agents enter the body through nose or mouth and attack the nervous system, subsequently stopping it from working.