The British Police are trying to identify the substance that rendered a man – believed to be a former Russian double agent – critically ill in Salisbury city. Although authorities did not reveal any identities, unidentified officials on Tuesday said Sergei Skripal and his daughter Yulia were undergoing treatment at a hospital after being exposed to an unknown substance, Reuters reported.

Skripal and the 33-year-old woman were found unconscious on Sunday on a bench outside the Maltings shopping centre. The Wiltshire Police said the two did not have any visible injuries, the BBC reported. Authorities ruled out any threat to the public because of exposure to the unknown substance, but they sealed the area and cordoned off a pizzeria.

In 2006, Moscow 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.

UK Foreign Secretary Boris Johnson said his country would respond “robustly” if it found that Russia was behind Skripal’s condition, the BBC reported. Although he described Russia as “a malign and disruptive force”, Johnson said he was not accusing anyone at this stage of the investigation.

“I say to governments around the world that no attempt to take innocent life on UK soil will go unsanctioned or unpunished,” the foreign secretary told MPs, even as Moscow denied any involvement.

Johnson said “it would be very difficult to imagine” that the United Kingdom’s participation in the football World Cup would proceed in the “normal way”. His associates, however, said the foreign secretary was referring to “officials” – and not the England team, BBC reported.

Mark Rowley, Britain’s top counterterrorism officer, said investigators need to be open to the possibility of “state threats”, referring to the murder of Alexander Litvinenko, an outspoken critic of Russian President Vladimir Putin who had sought refuge in the UK. An investigation by British authorities had found that Moscow was behind Litvinenko’s polonium poisoning in 2006. Russia has denied the allegations.

On Tuesday, the Kremlin said Britain had yet to ask for its help in the investigation, but “Moscow is always open for cooperation”.