LONDON—Fears of another Kremlin-sanctioned assassination attempt on British soil deepened Tuesday when it emerged that several emergency responders were hospitalized after dealing with an unconscious Russian spy suspected to have been exposed to an unknown substance.

An Italian restaurant and a pub in the center of the cathedral city of Salisbury remained cordoned off as detectives investigate what happened to Sergei Skripal, 66, who collapsed in broad daylight Sunday along with a younger woman who is believed to be his daughter.

Skripal is a former Russian intelligence officer who was jailed in his homeland for spying for MI6 and allowed to relocate to the West after a spy-swap that included sending the infamous Russian spy Anna Chapman back to Russia.

Foreign Secretary Boris Johnson said there were clear “echoes” of the poisoning of Alexander Litvinenko—who was dosed with a radioactive isotope in London—although he said it was too early to point fingers despite the “suspicions.” He said Britain would “respond appropriately and robustly” should evidence emerge linking Moscow to the sudden illness of the Russian double-agent.

British newspaper reports claim that he had been concerned for his safety before he lost consciousness in a busy riverfront shopping district Sunday afternoon—the BBC reported that the younger woman was his daughter, Yulia Skripal, who was visiting Britain from Russia.

The pair remain in a critical condition and are being treated for exposure to an unknown substance.

The authorities were so concerned about the nature of that substance that a restaurant and pub remain closed to the public and a small number of emergency workers were taken to hospital to be assessed. A Wiltshire police spokesman said one of those first responders was still in the hospital.

Wiltshire police say they have not yet confirmed that a crime has taken place but their response to the discovery of the potential victims suggests they are taking that possibility seriously. Officers sealed off the area and later shut down the emergency room at the hospital for fear that the suspected substance could affect other people.

Skripal was a Russian colonel who worked for the army inteligence unit known as the GRU. He was arrested in 2006 for passing information to the British about Russian spies dating back to the 1990s. The FSB, the notorious successor to the KGB, claimed that Skripal was recruited by the British in the mid-1990s and was paid as much as $100,000 into a bank account in Spain for a decade’s worth of spying.

He retired from the GRU in 1999 before joining the Russian Foreign Ministry until 2003. In a trial three years later, the Russians say they discovered that he had been shopping information on its spies stationed all over Europe.

He was sentenced to 13 years in prison in 2006, but he was released early and allowed to retire in Britain’s picturesque southwest as part of a major spy swap. At the time of his release, his lawyer said Skripal was concerned about his health as he was suffering from diabetes.

Until British authorities have obtained toxicology reports it will remain unclear whether Skripal’s collapse raises suspicions about Russian influence.

In previous examples of suspected Russian assassinations in Britain, the poison has not been detected until long after the victims had sought medical assistance.

Former FSB officer Litvinenko died in agony in a North London hospital after polonium-210 was slipped into a pot of tea at a Mayfair hotel by two Russian agents, Andrei Lugovoy and Dmitry Kovtun.

It was weeks before the authorities identified the highly unusual radioactive poison and many more years until an official inquiry formally found that Litvinenko had “probably” been murdered on the orders of the Kremlin.

The case of Alexander Perepilichnyy, a Russian whistleblower who collapsed while running near his home in the suburbs outside London, was even more embarrassing for the British authorities.

Local police initially believed that he had died of natural causes, but a shock toxicology report later suggested the hand of an assassin. Traces of the deadly Gelsemium elegans flower, which is a known weapon of Chinese and Russian contract killers, were found in his stomach.

Perepilichnyy had been due to testify about Russian corruption in Switzerland.