Puzzle lacking vital piece: who would kill Mr Williams and why?

Coroner scotches transvestite theory and rules out link to MI6

Coroner Dr Fiona Wilcox acknowledged that there was no sign of a break-in or robbery (Nick Razzell)

The coroner's conclusion that Mr Williams was probably a victim of unlawful killing is marked by a crucial absent factor – a motive.

In reaching her decision Dr Wilcox ruled out that the death had any connection with either Mr Williams's professional or personal life. She accepted evidence, she said, that his job as a computer expert with MI6 was "low risk" and would not have made him a target.

The inquest had heard that Mr Williams had visited bondage websites and had once been found by his landlady in Cheltenham tied to his bedpost wearing boxer shorts and nothing else. After his death more than £2,000 of female designer clothes and around 26 pairs of ladies' shoes were found in his flat.

Dr Wilcox pointed out that the MI6 officer had only visited bondage sites four times. There was no female underwear present, and no evidence he was a transvestite, she maintained. The female clothes were likely to have been part of a "fashion collection" as he was interested in fashion.

The coroner also accepted evidence that Mr Williams was ultra-cautious over his job and would not have let people he did not know into his flat. At the same time she acknowledged that there was no sign of a break-in or a robbery. So the obvious question remains – who killed Mr Williams and why?

The presence of a third party at Mr Williams's flat should have yielded forensic evidence. But only minute traces of DNA belonging to someone else were found. Yet Detective Chief Inspector Jackie Sebire, the officer in charge of the murder investigation, stated that there was no evidence of a clean-up.

The decomposition of the body was speeded up by the heating being turned on. Dr Wilcox acknowledged yesterday that August 2010, when Mr Williams died, was a "hot August, not a cold one". Yet there is no explanation as to why the heating was on.

The investigation into the death was undermined by a series of blunders, and also allegations of a cover-up. However, although MI6 acknowledges that the failure of Mr Williams's superior to raise the alarm over his absence from work was a mistake, the agency is adamant that subsequently all help asked for was given to the police. The agency was criticised in court for not turning over pieces of potentially important evidence to the murder squad. It later transpired that it was the police's SO15 department which had failed to pass on the material which had been delivered by the intelligence service. It has not been explained why this happened.

Last night an MI6 spokesman said: "We fully co-operated with the police and will continue to do so. We gave all the evidence to the police when they wanted it; at no time did we withhold evidence."

The inquest had heard that Mr Williams had agreed to forge an education certificate for a Kurdish acquaintance and some work for this was found on one of his computers. The inquest accepted evidence from a friend of Mr Williams, Elizabeth Guthrie, that this was a light-hearted idea which was never proceeded with. Yet the Kurdish man was not called to give evidence.

The inquest was also told that Mr Williams had carried out unauthorised trawling of the MI6 website. The details of this did not emerge, the coroner accepting witness testimony that "there was less to this than meets the eye" – yet another piece of the puzzle in the death of the MI6 officer.