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Secret agents specialising in the "dark arts" might have tried to cover up the death of an MI6 spy found in a holdall, a coroner has heard.
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."
The lawyer for family of Gareth Williams has made a dramatic intervention at the pre-inquest review into death of MI6/GCHQ code breaker.
He's says there is no DNA and no fingerprints from anyone else in the flat and there are no signs of covert entry .
A lawyer for the police said he was worried about "sensationalising" the inquest with a live demonstration of the bag and offers a video clip by an expert as alternative.
Police reveal "problems in the DNA analysis" of mixed profile found on Williams' hand - found the contamination was from scientist at scene.
The error was spotted by a scientist who's evidence helped convict the killers of Stephen Lawrence in January. The coroner wants to hear from her.
A Mediterranean couple sought in connection with Williams' death is now considered a "red herring" after further police inquiries.
Organisers of the ladies fashion course Williams' enrolled on in secret will give statements to the inquest too.
The lawyer for MI6 says calling corporate witnesses will "paint a picture and set the scene".
Two experts have tried to reconstruct how Mr Williams could have got into the bag and locked it - The coroner says he wants a live demonstration in court.
The issue of whether or not Mr Williams could have got into the bag and padlocked it from the inside "goes to heart" of inquest the coroner says.
A pre-inquest review into the death of MI6 and GCHQ code breaker Gareth Williams, who was found dead in a holdall in his London flat in August 2010, is deciding on witnesses to call.
They will include Gareth's sister and the police officers who opened the front door and discovered the padlocked bag containing his body.
DCI Jackie Sebire who led investigation will also give evidence and will explain photos taken inside the flat and CCTV of Williams's movements.
An officer from the counter terrorism command will also attend the inquest to explain its role in the police after Williams' family asked for it.
Evidence will also be given about the contents of Williams's locker at GCHQ, which several people had access to.
Latest ITV News reports
A coroner has been told that secret agents specialising in so-called "dark arts" may have tried to cover up the death Gareth Williams.