Relatives of an MI6 spy whose body was found in a holdall will demand to find out if he was killed by secret services as an inquest opens on Monday.
Fellow agents, police and friends are giving evidence about Gareth Williams in a bid to help a coroner solve the 21-month mystery.
Family members fear "some agency specialising in the dark arts" leaves them with no way of knowing how and why he died.
Scotland Yard has drawn a blank in its bid to explain whether he died at the hands of a third party.
Relatives believe someone was either present when he died or broke into his home afterwards to destroy 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 Williams was alive inside the bag and locked it himself was "at the very heart of this inquiry".
The naked and decomposing body of Williams, 31, was found in the bath of his home in Pimlico, central London, in August 2010.
The discovery sparked a painstaking investigation, worldwide media frenzy and conspiracy theories.
Williams, of Anglesey, north Wales, was found in a large North Face holdall, sealed by a padlock, at his top-floor flat in Alderney Street.
Postmortem tests failed to determine how he died and police originally found it would have been impossible for him to have locked himself inside.
Family lawyer Anthony O'Toole has said the inquest at Westminster coroners 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 postmortem by experts in those dark arts."
The mathematics prodigy worked as a cipher and codes expert for GCHQ, the government listening station, but had been on secondment with MI6 since March 2010.
O'Toole said: "In our submission, to properly explore the circumstances of the death we need to establish the deceased's work."
Relatives want to know why the alarm was not raised when 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.
It emerged in March that two areas of investigation were red herrings. Forensic examiners mistakenly flagged up a spot of DNA on Williams's hand in 2010, before realising six weeks ago that it matched a scientist on the crime scene.
It also emerged that a Mediterranean couple police wanted to speak to were irrelevant to Williams's death.
Wilcox has indicated she may want to see a practical demonstration of how Williams might have got into the bag and locked it himself.
Experts agreed locking the bag from the inside "would have been very difficult, if not impossible", Metropolitan police lawyer Vincent Williams has said.
The inquest will hear that Williams may have died after breathing too much carbon dioxide.