Prosecutor Sue Hemming said Wednesday that the U.K. is not asking Moscow to extradite the men because Russian law forbids extradition of the country’s citizens.
Police say the men, both about 40, flew from Moscow to London on Russian passports two days before the Skripals were poisoned on March 4.
Assistant Police Commissioner Neil Basu said the men were probably using aliases.
Basu would not say whether police believe the suspects worked for Russian security services but added: “This was a sophisticated attack across borders.”
British officials have blamed the Russian government for the poisoning, a charge Moscow has denied.
Police released a series of images of the men as they traveled through London and Salisbury between March 2 and March 4. Police say the two men flew back to Moscow from Heathrow Airport on the evening of March 4, hours after the Skripals were found collapsed on a park bench in Salisbury.
Basu said traces of Novichok contamination were found in the London hotel room where the two men had stayed.
“Tests were carried out in the hotel room where the suspects had stayed. Two swabs showed contamination of Novichok of levels below that which would cause concern for public health,” Basu said.
Police said they believe the nerve agent was smuggled to Britain in a counterfeit Nina Ricci perfume bottle and applied to the front door of the ex-spy’s house.
More than three months later, the bottle was found by a local man, Charlie Rowley. He was hospitalized and his girlfriend Dawn Sturgess died after being exposed to the contents.
Police are still trying to determine where the bottle was between the Skripal poisoning in March and its discovery by Rowley on June 27.
As a result, he said, police are not yet ready to press charges in the second poisoning.
Britain has issued a European Arrest Warrant for the suspects, meaning they can be detained if they leave Russia for another European country, but Basu conceded it was “very very unlikely” police would be in a position to arrest them any time soon.
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