Gareth Wyn Williams (26 September 1978 – c. 16 August 2010) was a Welsh mathematician and employee of GCHQ seconded to the Secret Intelligence Service (SIS or MI6) who was found dead in suspicious circumstances at a Security Service safe house flat in Pimlico, London, on 23 August 2010.[1] The inquest found that his death was "unnatural and likely to have been criminally mediated." A subsequent Metropolitan Police re-investigation[2] concluded that Williams's death was "probably an accident".[3] It is also suspected that he should be numbered amongst those who have been killed by Russian assassins on British soil. Two senior British police sources have said some of Williams's work was focused on Russia – and one confirmed reports that he had been helping the NSA trace international money-laundering routes that are used by organised crime groups including Moscow-based mafia cells.[4]
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https://www.telegraph.co.uk/news/uknews/crime/7969595/Gareth-Williams-backroom-boy-spy-was-really-a-high-flier.html