The attack on Sergei Skripal, 66, and his daughter Yulia, 33, is being treated as attempted murder, Metropolitan Police counterterrorism chief Mark Rowley told reporters at a news conference at Scotland Yard on Wednesday.
Police believe that they were specifically targeted, Rowley said. The two were in critical condition after being found unconscious Sunday on a bench in Salisbury, 90 miles west of London.
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Rowley said he could not offer any specifics about the attack, including the type of nerve agent used or how it was delivered.
Skripal, a former military intelligence officer, was sentenced to 13 years in prison in 2006 after being convicted in Russia of spying for Britain.
He passed the identity of dozens of spies to the United Kingdom's MI6 foreign intelligence agency, according to news reports. He was freed in 2010 as part of a U.S.-Russian spy swap that also included Anna Chapman, who was arrested in New York earlier that year.
The incident has drawn parallels to the death of former Russian agent Alexander Litvinenko, who was poisoned with radioactive polonium 11 years ago in London.
"I think we have to remember that Russian exiles are not immortal, they do all die and there can be a tendency for some conspiracy theories," Rowley told the BBC.
Litvinenko, 43, an outspoken critic of Russian President Vladimir Putin, fled Russia for Britain six years before he was poisoned. He died after drinking green tea laced with the rare and very potent radioactive isotope at London’s Millennium Hotel.
In a report published in 2016, a British judge found that Litvinenko was killed in an assassination carried out by Russia's security services — with the probable approval of Putin. Russia has denied any responsibility for Litvinenko's death.
There is no evidence of any Kremlin connection in Skripal's case. But intelligence analyst Glenmore Trenear-Harvey, who formerly worked for MI6, told NBC News that he believes that the case has the hallmarks of Putin's involvement.
Symptoms of exposure to nerve agents may include respiratory arrest, heart failure, twitching or spasms, according to Malcolm Sperrin, a fellow of the Institute of Physics and Engineering in Medicine in the U.K.
Yuliya Talmazan and Michele Neubert contributed reporting from London, and Keir Simmons and Nick Bailey from Salisbury, England.
Yuliya Talmazan, Michele Neubert, Keir Simmons and Nick Bailey contributed.