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LONDON — British prosecutors have charged two Russian men with the nerve agent poisoning of ex-spy Sergei Skripal and his daughter Yulia.

The Crown Prosecution Service said Wednesday that Alexander Petrov and Ruslan Boshirov are accused in absentia of conspiracy to murder, attempted murder and use of the nerve agent Novichok.

British Prime Minister Theresa May later said that U.K. officials had concluded that the men work for Russia's GRU military intelligence agency. The Kremlin has denied involvement in the case, but May said Russia was spreading "lies."

The U.K. will not ask Moscow to extradite the men because Russian law forbids the extradition of the country's citizens, prosecutor Sue Hemming told reporters.

Alexander Petrov and Ruslan Boshirov have been charged in connection with the nerve agent poisoning of ex-spy Sergei Skripal and his daughter Yulia. Metropolitan Police / AP

Skripal is a former colonel in Russian military intelligence who betrayed dozens of agents to Britain's MI6 foreign spy service. He was found unconscious on a public bench along with his daughter in the English city of Salisbury on March 4.

Police say the men, who are both aged about 40, flew from Moscow to London on Russian passports two days before the Skripals were poisoned.

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Assistant Police Commissioner Neil Basu said the men were probably using aliases.

Speaking before May's comments, Basu would not say whether police believe the suspects worked for Russian security services but described the incident as "a sophisticated attack across borders."

Russian Foreign Ministry spokeswoman Maria Zakharova said Moscow had no knowledge of the suspects identified by U.K. police, adding that their names and photos "say nothing to us."

Zakharova called on Britain to cooperate with Russian law enforcement agencies on the investigation. She has criticized London for turning down Moscow's request to see the case files.

Police released a series of images of the men as they traveled through London and Salisbury between March 2 and March 4.

Investigators 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 the park bench.

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.

Novichok was developed by the Soviet military in the 1970s and 1980s.

The counterfeit perfume box that British police say Novichok was transported in. Metropolitan Police / AFP - Getty Images

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, Basu 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.