Gareth 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. The inquest found that his death was "unnatural and likely to have been criminally mediated." A subsequent Metropolitan Police re-investigation concluded that Williams's death was "probably an accident."