“This guy is not a big critic,” Marina Litvinenko, the widow of the whistle-blower, said of Mr. Skripal, speaking by telephone from an undisclosed location outside London. “Everyone says he kept a low profile.”
And while Russia’s likely involvement in the death of Mr. Litvinenko was documented in a long-running British inquiry, the illness that struck Mr. Skripal and his daughter, Yulia, remains mysterious.
The British inquiry concluded two years ago that an operation by the F.S.B. to kill Mr. Litvinenko, 43, by poisoning him with polonium slipped into his tea in an upscale London hotel, “was probably approved” by President Putin.
Yet Britain took relatively modest countermeasures, and the two men accused of the killing, Andrei K. Lugovoi and Dmitri Kovtun, remain at large in Russia despite British demands for their extradition.
“If it was poison, it would mean people are still not protected,” Ms. Litvinenko said, referring Mr. Skripal’s case. “It’s a very bad message for other people. The lessons from my husband’s death have not been properly learned.”
Bill Browder, a wealthy American fund manager clamoring for justice on behalf of his lawyer, Sergei L. Magnitsky, who died in a Russian prison in 2009, told the Press Association news agency that “the first operating assumption” of the Skripal case should be “that this was an assassination attempt by the Kremlin against a traitor of Russia.”
That is not improbable. The latest episode fits a broader pattern of mysterious illnesses — sometimes deaths — among Russians seen as enemies of the Kremlin who are living in Britain, where the presence of wealthy, high-rolling Russians has given London one of its newer nicknames: Londongrad.
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Mr. Skripal, 66, was arrested in 2004 for selling Russian secrets to British intelligence officers, possibly including the identities of Russian spies in Europe. Even after he retired from active duty in 1999, according to British news reports, he continued to tease out salable information from former colleagues.
The chronology suggests that Mr. Skripal was spying against the F.S.B. at precisely the moment that Mr. Putin, a former K.G.B. officer on his rise to supreme power, headed the organization. Treachery was not something he tolerated.
“I’m sure Putin regards him as a traitor,” said Tony Brenton, the British ambassador in Moscow from 2004 to 2008 — a period of rapidly deteriorating relations that some compared to the Cold War.
But, Mr. Brenton added in an interview, “the evidence isn’t there yet” about why Mr. Skripal fell ill. Investigators in the Litvinenko case discovered shortly before he died that polonium had been used, Mr. Brenton said, noting that if Mr. Litvinenko had passed away earlier, “they would never have found it.” The killers in that case, he added, “could have gotten away with it.”
Mr. Skripal, once a colonel in Russia’s military intelligence service, the G.R.U., was arrested in 2004 and jailed in 2006, accused of selling the Kremlin’s secrets to Britain’s MI6 overseas intelligence service over a 10-year period in return for $100,000 paid into a bank account in Spain. Mr. Litvinenko’s widow said that he, too, had received a stipend from MI6, but only after the family moved to London in 2000.
The authorities have rarely moved swiftly to follow suggestions that the Kremlin is actively pursuing a policy of reprisals against adversaries beyond its own borders. In 2012, for instance, Alexander Perepilichnyy, a whistle-blower financier, died near his home outside London. An inquiry by his life insurance company found traces of a rare and toxic plant extract — gelsemium — in his stomach, but the British police have insisted that there was nothing suspicious.
“British law enforcement doesn’t have a lot of credibility when it comes to poisoned Russians in the U.K. after the complete failure of finding out what happened to Perepilichnyy in 2012,” Mr. Browder said on Twitter.
The public inquiry into Mr. Litvinenko’s death came even as British officials fought strenuously to block a deeper look into the circumstances, including using security legislation to seal some details from public scrutiny.
The head of the Home Office at the time was Theresa May, who is now the prime minister of country that must find a response after the death of another Russian on British soil.