MI6 officer Gareth Williams, whose body was found in a padlocked bag, had complained of "friction" at the service and wanted to leave, says his sister on the first day of an inquest into his death.

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The inquest opened today by Coroner Fiona Wilcox will examine whether another person was involved in Gareth Williams' death and will hear from 37 witnesses. The naked and decomposing body of the MI6 agent was found in a padlocked bag in the bath of his home in Pimlico, central London, in August 2010.

Williams' sister Ceri Stubbs said he was "the most scrupulous risk assessor" she had ever known. "I cannot emphasise enough his conscientiousness," she said, adding that she did not believe Mr Williams would let a potential killer into his upmarket London flat.

In a statement, Ms Stubbs said that her brother hated the "rat race" and the "flash car competitions" at MI6, and was due to return to the west country a week after he was found dead.

Mathematics prodigy Mr Williams worked as a cipher and codes expert for GCHQ, the government listening station, but had been on secondment with MI6 since March 2010. MI6 were "dragging their feet" in approving his request in April 2010 to return to GCHQ's Gloucestershire HQ, she added.

The coroner leading the investigation into the "highly controversial" death of the MI6 agent said all evidence would be in the public realm, but that four spies could remain anonymous. Some information which compromises national security will be heard in private, but Dr Wilcox said that a summary of any withheld evidence would be presented in court.

Charges still a 'possibility'

The lawyer for the Metropolitan Police tried to block a coroner from publishing video footage which could be key to a prosecution, arguing that charges were still a "real possibility" and that disclosure of evidence could compromise criminal proceedings. A "careful line must be struck between open justice" at the inquest and a criminal investigation, said Vincent Williams.

However Dr Wilcox asked him to reach a compromise with the lawyer representing the media, who asked that photographs, video and documents referred to in open court be supplied to the broadcasters and national newspapers.

The inquest began with a summary of evidence heard in the pre-inquest review, including claims that Mr Williams' death may have been a cover-up by secret services. Dr Wilcox expressed sympathy to family members, who fear that "some agency specialising in the dark arts" which would leave them with no way of knowing how and why he died.

'Friction' at MI6

In her testimony, Ms Stubbs painted a picture of an "exceptional" academic who was a private, very tidy man who never talked about his work.

"He disliked office culture, post-work drinks, flash car competitions and the rat race. He even spoke of friction in the office," she said in a statement, adding: "The job was not quite what he expected. He encountered more red tape than he was comfortable with."

He disliked office culture, post-work drinks, flash car competitions and the rat race. He even spoke of friction in the office. Gareth Williams's sister

When asked about the £20,000 of women's clothes which were found in his flat after he died, she said it was "not particularly" surprising, adding that they were "possibly as a gift".

"In terms of a big brother figure, Gareth was perfect," she said. "It's impossible to do justice to Gareth's impressive character without meeting him.

"As a family we were incredibly close."

The lawyer representing Gareth Williams' family said they objected to a video showing an expert trying to lock himself in the bag which the spy's body was found, being released to the media. Anthony O'Toole said it could be "upsetting" for Mr Williams' relatives if the "speculative" film was released more widely.

"The family are anxious to ensure that that reconstruction does not become bandied about the various media outlets, both in this country and abroad," said Mr O'Toole. "We are anxious as a family not to do anything to impede somebody in due course, hopefully, being brought to account for this - if there is somebody who should be brought to account."

Unanswered questions

In the 21 months since his death, the Metropolitan Police has been unable to determine whether he died at the hands of a third party.

Relatives are keen to find out 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

At the inquest, Dr Wilcox said she may want to see a practical demonstration of how Mr Williams might have got into the bag and locked it himself.