British police said Wednesday that a Russian ex-spy and his daughter, who collapsed near a shopping mall over the weekend in southern England, were exposed to a nerve agent, adding to suspicions of a Kremlin connection to the poisoning.
"Having established that a nerve agent is the cause of the symptoms leading us to treat this as attempted murder, I can also confirm that we believe that the two people who became unwell were targeted specifically," Metropolitan Police counterterrorism chief Mark Rowley said at a news conference in Salisbury.
He did not elaborate on the nature of the nerve agent.
Forensic experts were continuing to search the area where Sunday's attack took place, but Sally Davies, England's chief medical officer, has said the risk of contamination was low for the general public.
Police initially said that Sergei Skripal, 66, and his daughter, Yulia, 33, who both remain in critical condition, had been exposed to an "unknown substance." They were found collapsed on a bench in Salisbury.
Skripal had retired from Russia's SVR foreign intelligence service, a successor to the KGB, when he was arrested and convicted in 2006 of working undercover for Britain's MI6. In 2010 he arrived in the U.K. as part of a prisoner swap between Moscow and Washington.
Although the nature of the Skripals' illness was not immediately known, its unusual nature immediately led to comparisons with the death of Alexander Litvinenko, a former Russian intelligence officer who defected to Britain and died in 2006 after being exposed to polonium-210, a rare radioactive isotope.
In 2016, an inquiry into Litvinenko's death concluded that it had probably been ordered by Russian President Vladimir Putin.
On Tuesday, British Foreign Secretary Boris Johnson, speaking in the House of Commons, said the mysterious poisoning of Skripal carried "echoes of the death of Alexander Litvinenko in 2006."
The Guardian newspaper reports that it is "unusual [for Russia] ... to target spies after they have been swapped. One possible reason is that Skripal was being punished for a continuing relationship with British intelligence, or the suspicion of one."
"'My presumption is that if the Russians were behind this, and it does look plausible, then it is because they assumed Skripal was still working for British or other western intelligence and not simply retired,' said Mark Galeotti, a Russia watcher and security analyst. 'That is likely what tipped the balance with Litvinenko.' "