Conclusion.

Taken from http://www.sherlock-holmes.co.uk/

Scientists had succeeded in discovering that it was ricin that had killed Markov, but investigators fared less well in finding the bearer of the umbrella.  Diplomats at the Bulgarian Embassy in London described allegations that their secret police had been involved as ?absurd?, but evidence indicated otherwise. 

A year earlier, another Bulgarian émigré, Vladimir Kostov was mysteriously jabbed outside a metro station.  He had fallen sick, but subsequently recovered.  Later, a surgeon extracted a small metal pellet from his flesh which was later shown to be identical to the one which had killed Markov.  Kostov had survived because the pellet had lodged itself in muscle in his upper back, away from major blood vessels.

It was not until the collapse of communism in 1991 did Bulgaria?s new government admit that its predecessors had ordered assassinations of Bulgarian defectors, including Georgi Markov.  The individuals responsible have not been identified.