MARY LOUISE KELLY, HOST:

London, 1978, Bulgarian dissident Georgi Markov stabbed with a poison-tipped umbrella on a bridge crossing the Thames. London, 2006, ex-KGB officer Alexander Litvinenko dead after drinking tea from a teapot contaminated with radioactive polonium. Surrey, southeast England, 2012, Russian banker-turned-whistleblower Alexander Perepilichny dead under mysterious circumstances, although an autopsy detected traces in his stomach of a chemical found in the rare poisonous plant gelsemium - all people who crossed the Kremlin and then met an untimely end on British soil, and all cases which may, or may not, prove relevant background to this next story which involves another former Russian spy found this past weekend slumped on a park bench in Salisbury, southern England.

BBC's security correspondent Gordon Corera is on the line from London. And, Gordon, who is this ex-spy? What do we know about him?

GORDON CORERA: Sergei Skripal is a Russian intelligence officer, a former one, who was in the Russian military intelligence, rose up to the rank of colonel but, it was alleged, had started to spy for Britain's intelligence service MI6. He was arrested in 2004 and put on trial in 2006...

KELLY: In Russia.

CORERA: ...Where he was accused of selling secrets, such as the identity of Russian undercover operatives, to British intelligence. And he was sentenced to 13 years in jail. He was serving that sentence until July 2010, when he was swapped as part of a rather dramatic spy swap largely involving the U.S. and Russia when that group of Russians, so-called illegals, was discovered by the FBI.

KELLY: Yes. This was the famous - redheaded Anna Chapman is the one who Americans might remember.

CORERA: Anna Chapman. Indeed. And the Russians wanted this group of their undercover operatives back, and the U.S. and U.K. appeared to strike a bargain in which they got a number of people out who were serving time in Russian prisons in exchange. And one of them was this man Sergei Skripal who then came to the U.K. where he was living very quietly until just the last few days.

KELLY: OK. So he's been living in the U.K. He turns up unconscious on a park bench this past Sunday. What happened?

CORERA: Well, members of the public saw him and the young women, who we now know to be his daughter, looking clearly unwell. I mean, she was slumped. He looked like he was perhaps hallucinating - strange arm movements. And the members of public called the police. By the time police came, they were unconscious. And quite quickly, it became clear this was something unusual - because by the next morning, the hospital had declared a major incident, which was a sign that they were dealing with what they described as an unknown substance and that these two people had been exposed to it. Still not clear what that substance might be, but, clearly, that caused great alarm - combined with the identity of this man and the fact he was a former Russian intelligence officer.

KELLY: OK. So for the record, it's possible that they are sick with something completely banal, that this has nothing to do with international espionage. That hasn't been ruled out. What evidence is there that Russia may have played a role, that there was some foul play happening here?

CORERA: I think particularly at the start, there was some question, could this just be an illness? But then today, the U.K.'s counter-terrorism command took over the investigation. So clearly, the focus is increasingly that this has a strong national security dimension. There is no, as of yet, hard evidence saying this is certainly the Russian state but means and motive. Means? This kind of poisoning with an unknown substance - that is not something criminals tend to do, but it is something the Russian state has a track record of doing. Motive? Well, this was a former Russian intelligence officer who would be viewed by his former colleagues as a traitor. And then you, of course, have this context of things like the Litvinenko case in 2006. So for those reasons - even though we do not have yet any hard evidence proving a link to the Kremlin, you can see why all the speculation is moving towards that link to the Russian state.

KELLY: Gordon, thank you.

CORERA: Thank you.

KELLY: The BBC's Gordon Corera. Transcript provided by NPR, Copyright NPR.