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Secret agents specialising in the "dark arts" might have tried to cover up the mysterious death of an MI6 spy found in a holdall, a coroner heard today.
Relatives believe a third party was either present when Gareth Williams died or broke into his home afterwards to destroy evidence, lawyer Anthony O'Toole said.
The family is demanding answers after Scotland Yard revealed a key line of its inquiries had been an 18-month DNA mix-up.
Mr O'Toole told a pre-inquest review that Westminster Coroner's Court must establish why there was no evidence of another person in his London apartment when he died.
He said: "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 the dark arts."
Mr O'Toole said Mr Williams "could have been actively deployed" as an agent up to five months before his death.
"In our submission, to properly explore the circumstances of the death, we need to establish the deceased's work," the lawyer said.
Mr O'Toole said relatives wanted 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.
Fresh questions over the cause of his death were raised after it emerged that two areas of investigation were red herrings.
Forensic teams mistakenly flagged up a spot of DNA on Mr Williams's hand in 2010 - before realising just two weeks ago that it matched a scientist on the crime scene, the force told the review.
It also emerged that a Mediterranean couple police wanted to speak to were irrelevant to Mr Williams's death.
Coroner Fiona Wilcox expressed frustration over delays as "an ongoing problem" as she called for inquiries into the DNA "error".
Dr Wilcox said that whether Mr Williams was alive inside the bag and locked it himself "was at the very heart of this inquiry".
She also told the hearing she was keen to see a practical demonstration of how Mr Williams might have got into the bag and locked it himself.
Regarding the error by forensic teams, Ms Wilcox said: "I do not want it to overwhelm - it's a question of proportionality."
Detective Chief Inspector Jackie Sebire told the court the DNA evidence was previously regarded as a "key line of inquiry".
She said: "It came to our attention that there had been a very significant DNA finding on Gareth's hand... That was sent to LGC Laboratory."
But it emerged just weeks before today's hearing "that actually the DNA evidence was contamination by a scientist at the scene", the officer said.
The error arose because "two numbers were put in the wrong way round" on a computer.
The force launched a review of the handling of other forensic evidence after the mistake was discovered.
The naked and decomposing body of Mr 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 several outlandish conspiracy theories.
Mr 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.
A battery of post-mortem tests failed to determine how he died and police originally found it would have been impossible for him to have locked himself inside.
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.
Experts agree that locking the bag from the inside "would have been very difficult, if not impossible", Metropolitan Police lawyer Vincent Williams said.
Mr Williams said the evidence suggested there is no need for a demonstration as to how the spy might have got in the bag.
The inquest, due to start next month, will hear that Mr Williams may have died after breathing too much carbon dioxide.
The remainder of the hearing was held behind doors closed to the public.
PA