LONDON - Sergei Skripal, a former Russian double agent who came to Britain in a spy swap in 2010, lies in critical condition in hospital after being exposed to an unknown substance. His daughter is also critically ill after the incident in Salisbury, southern England.

Police say they are still investigating whether a crime has been committed.

But Skripal is not the first opponent of the Kremlin to be struck down in Britain in mysterious circumstances.

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BORIS BEREZOVSKY

A Russian oligarch who moved to Britain in the early 2000s and became a strident critic of Russian President Vladimir Putin, Berezovsky was found dead on a bathroom floor at his home in southern England in March 2013, with a scarf around his neck.

After an inquest, coroner Peter Bedford said it was impossible to conclude beyond a reasonable doubt whether Berzovsky was killed or committed suicide.

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ALEXANDER PEREPILICHNY

A Russian businessman who was a key witness against Russian officials accused of stealing $230 million from a London hedge fund, Perepilichny collapsed and died while jogging near his rented home south of London in November 2012.

Two autopsies failed to determine a cause of death and no known toxin was found, but colleagues believe he may have been poisoned with a hard-to-detect plant.

A coroner's inquest to determine the cause of death is ongoing.

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ALEXANDER LITVINENKO

A former KGB agent who had defected to Britain and become a vocal Kremlin critic, Litvinenko died in November 2006, three weeks after drinking tea laced with the radioactive isotope polonium-210 at a London hotel.

On his deathbed, he accused Putin of ordering his killing. The case chilled U.K.-Russian relations for years.

British police charged two Russian men, Alexander Lugovoi and Dmitry Kovtun, with killing Litvinenko, but Moscow refused to extradite them. A public inquiry concluded in 2016 that Litvinenko had been killed by Russia's security service, likely on Putin's orders. The Russian government has denied any responsibility.

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GEORGI MARKOV

A Bulgarian dissident working for the BBC, Markov died in September 1978, four days after he was jabbed in the thigh with a poison-tipped umbrella while waiting for a bus on London's Waterloo Bridge. The umbrella tip contained a tiny pellet of ricin, a toxin derived from caster beans that is lethal in tiny doses.

KGB agents and senior members of Bulgaria's secret police were suspected of being involved in the killing, but Markov's killers have never been brought to justice.

News from © The Associated Press, 2018