“I can prove they were here operating as a unit linked to the G.R.U.,” he said. “We remain as determined as ever to bring those responsible to justice.”

Mr. Sergeev was charged with conspiracy to murder, attempted murder and possession and use of a chemical weapon. The three suspects are in Russia, and there is little chance anyone involved in the poisonings of Mr. Skripal and Mr. Litvinenko will face trial.

In addition to Mr. Skripal and his daughter, a British police officer, Detective Sergeant Nick Bailey, who was among the first to respond to Mr. Skripal’s home, was also poisoned. All three survived, but a 44-year-old mother of three, Dawn Sturgess, later died when she sprayed herself with the contents of a perfume bottle that police later said was used to transport the nerve agent. Her boyfriend, who was also sickened and survived, had found the bottle, disguised to look like Nina Ricci Premier Jour perfume, in a garbage bin.

The new charges came on the same day that the European Court of Human Rights concluded that Mr. Litvinenko’s assassins in 2006 were acting as “agents of the Russian state.” The ruling bolstered a separate inquiry by Britain that found “strong circumstantial evidence” that Mr. Putin and his spy chief at the time, Nikolai Patrushev, had approved an operation to kill Mr. Litvinenko, using a dangerous and rare isotope, polonium 210.

The lengthy British inquiry concluded in 2016 that Andrei K. Lugovoi, a former K.G.B. bodyguard, and Dmitri V. Kovtun, a Red Army deserter, poisoned Mr. Litvinenko.

While the 328-page report was scathing, it cited no specific evidence that Mr. Putin or Mr. Patrushev had been aware of the plot to kill Mr. Litvinenko or had sanctioned it.

Russian authorities have been openly disdainful of claims that Russian intelligence operatives were involved in the poisonings. In 2018, after British authorities charged Mr. Chepiga and Mr. Mishkin and described their roles in the Skripal poisoning, the pair gave an interview to one of the Kremlin’s chief propagandists, claiming implausibly that they were tourists and hinting that they were in fact in a romantic relationship.

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