Relatives of an MI6 spy found in a holdall will demand to find out if he was killed by secret services as an inquests opens.
The naked and decomposing body of Gareth Williams, 31, was found in the bath of his home in Pimlico, central London, in August 2010.
The mysterious case has left family members fearing " some agency specialising in the dark arts " will leave them with no way of knowing how and why he died.
Scotland Yard has not been able to conclude whether he died at the hands of a third party or not.
Relatives believe someone was either present when he died or broke into his home afterwards to destroy the evidence.
Coroner Fiona Wilcox, who has already expressed frustration at police over DNA errors, is expected to hear from 30 witnesses over five days.
She says that whether Mr Williams was alive inside the bag and locked it himself "was at the very heart of this inquiry".
Dr Wilcox has indicated she may want to see a demonstration of how Mr Williams might have got into the bag and locked it himself.
The discovery of Mr Williams' body sparked a painstaking investigation, worldwide media frenzy and several outlandish conspiracy theories.
He was found in a large North Face holdall, sealed by a padlock, at his top-floor flat in Alderney Street.
Family lawyer Anthony O'Toole has said the inquest at Westminster Coroner's Court must establish why there was no evidence of another person in his London apartment.
He told a pre-inquest review: "The impression of the family is that the unknown third party was a member of some agency specialising in the dark arts of the secret services, or evidence has been removed post-mortem by experts in those dark arts."
The mathematics prodigy, from Anglesey, North Wales, worked as a cipher and codes expert for GCHQ, the Government listening station, but had been on secondment with MI6 since March 2010.
Relatives want to know why the alarm was not raised when Mr Williams initially failed to turn up to work.
By the time officers arrived at his flat, his body was so decomposed that evidence had been lost.
The inquest will hear that Mr Williams may have died after breathing in too much carbon dioxide.