LONDON (AP) — British officials announced Wednesday that they have charged two Russian men with the nerve agent poisoning of ex-spy Sergei Skripal and his daughter Yulia in the English city of Salisbury — though they held out little hope of being able to bring them to justice.

The Crown Prosecution Service said the men, who entered the country under the names Alexander Petrov and Ruslan Boshirov, are being charged in absentia with conspiracy to murder, attempted murder and use of the nerve agent Novichok.

Prosecutor Sue Hemming said the U.K. would not ask Moscow to extradite the men because Russian law forbids extradition of the country’s citizens. Britain has issued domestic and European arrest warrants for the suspects, meaning they can be detained if they leave Russia for another European country.

But assistant commissioner Neil Basu, head of counterterrorism at London’s Metropolitan Police, conceded it was “very, very unlikely” police would be in a position to arrest them any time soon.

Police say the suspects, both about 40, flew from Moscow to London on Russian passports two days before the Skripals were poisoned on March 4. Basu said the passports were genuine but the men were probably using aliases, and appealed the public “to come forward and tell us who they are.”

Sergei Skripal — a former Russian agent who had been convicted in his homeland of spying for Britain — and his daughter were found collapsed on a bench in the cathedral city of Salisbury, 90 miles (140 kilometers) southwest of London. They spent weeks hospitalized in critical condition and are now recovering in a secret location for their own protection.

British authorities and the international chemical weapons watchdog say the Skripals were exposed to Novichok, a type of military-grade nerve agent developed by the Soviet Union during the Cold War.

Britain blames the Russian government for the attack, a claim Moscow denies. The poisoning ignited a diplomatic confrontation in which hundreds of envoys were expelled by both Russia and Western nations.

Russian Foreign Ministry spokeswoman Maria Zakharova said the names and images of the suspects released by Britain “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.

Basu would not say whether police believe the suspects worked for Russian security services but, he said, “this was a sophisticated attack across borders.”

Police on Wednesday gave new details about what Basu called “one of the most complex investigations” the force had ever seen.

This combination photo made available by the Metropolitan Police on Wednesday Sept. 5, 2018, shows Alexander Petrov, left, and Ruslan Boshirov. British prosecutors have charged two Russian men, Alexander Petrov and Ruslan Boshirov, with the nerve agent poisoning of ex-spy Sergei Skripal and his daughter Yulia in the English city of Salisbury. (Metropolitan Police via AP)

The police force 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.

Police believe the nerve agent used to poison the Skripals was smuggled to Britain in a counterfeit Nina Ricci perfume bottle and applied to the front door of Sergei Skripal’s house.

More than three months later, the bottle was found by a local man, 48-year-old Charlie Rowley. He was hospitalized and his girlfriend Dawn Sturgess, 44, died after being exposed to the contents.

The Organization for the Prohibition of Chemical Weapons confirmed Tuesday that Rowley and Sturgess were also exposed to Novichok.

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 bring charges in the second poisoning.

___
By JILL LAWLESS , Associated Press

Nataliya Vasilyeva in Moscow contributed to this story.