How Boris Yeltsin got Tories to block probe into umbrella assassin

Wiltshire, suspected the capsule had contained ricin because the deadly chemical had been found in a pellet recently removed from another Bulgarian dissident, Vladimir Kostov.

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A few weeks earlier Kostov had been leaving the Arc de Triomphe Metro station in Paris when he heard a crack like an airgun report and felt a sting on the right side of his back.

He was wounded but recovered quickly. After Kostov heard about Markov's murder he came forward and British doctors recovered the pellet and found ricin inside.

Somehow the poison had not been released into his bloodstream. Dr Gall knew the only place ricin was produced was in the Laboratory 12 in Moscow set up by Grigory Mairanovsky - known as Dr Death.

But it was only when Kalugin came forward 15 years later that the link between Markov's murder and the Soviet Union was fully established.