MI6 code breaker Gareth Williams was probably killed but the "spy in the bag" case might never be solved after mistakes by investigators, an inquest has heard.
Mr Williams's relatives attacked failures by secret services and police after a coroner ruled "many agencies fell short" in their investigation of the holdall death riddle.
Fiona Wilcox said she was sure a third party locked the 31-year-old mathematics prodigy inside the red holdall, probably while he was still alive.
She criticised the 21-month investigation, saying it was unlikely the mystery "will ever be satisfactorily explained".
"The cause of his death was unnatural and likely to have been criminally mediated," she said during a two-hour narrative verdict at Mr Williams's inquest.
"I am therefore satisfied that on the balance of probabilities that Gareth was killed unlawfully."
As the eight-day hearing ended, relatives spoke out for the first time about their grief being "exacerbated" by MI6's "reluctance and failure" to assist the police inquiry.
In a statement read out by their solicitor, they said they were "extremely disappointed" at "total inadequacies" in the inquiry.
Scotland Yard vowed to explore new evidence that has come to light, while Sir John Sawers, chief of the Secret Intelligence Service, apologised "unreservedly" for delays in raising the alarm about the death.
Mr Williams, a fitness enthusiast originally from Anglesey, North Wales, was found naked, curled up in the padlocked holdall in the bath of his flat in Pimlico, central London, on August 23 2010.
Pathologists said he would have suffocated within three minutes if he was alive when he got inside the 80 centimetres by 48 centimetres bag.
The coroner agreed that Mr Williams was suffocated by carbon dioxide, possibly as an onset of a short-acting poison.
She dismissed speculation that Mr Williams died as a result of some kind of "auto-erotic activity", also denying there was any evidence to suggest claustrophilia - the love of enclosed spaces - was of any interest to him.
His sister, Ceri Subbe, looked on as Dr Wilcox told a packed Westminster Coroner's Court that it "remained a legitimate line of inquiry" that the secret services may have been involved in the death.
But she said "there was no evidence to support" that he died at the hands of spies.
Despite a 21-month police inquiry and seven days of evidence, "most of the fundamental questions in relation to how Gareth died remain unanswered", she said.
Dr Wilcox said several factors hampered inquiries, including breakdowns in communication by her own coroner's office, a DNA mix-up by forensics and the late submission of evidence by MI6 to police.
She went on to question why details of Mr Williams's private life were leaked to the press.
The coroner ruled out Mr Williams's interest in bondage and drag queens as having any bearing on the death, before adding: "I wonder if this was an attempt by some third party to manipulate the evidence."
The lack of hand and footprints in the bathroom was "significant", Dr Wilcox said, telling the court: "In relation to the prints found within the bathroom, in my view what was more significant was what was not found rather than what was found."
Dr Wilcox found it "highly unlikely" that Mr Williams died alone, saying: "If Gareth had been carrying out some kind of peculiar experiment, he wouldn't care if he left any foot or fingerprints."
She said the "highly unusual circumstances" of Mr Williams's death immediately raised the possibility of foul play, which had prompted "endless speculation".
But the coroner added that "taking all these shortfalls together, I am satisfied that the evidence is reliable and that we do not have to adjourn at this point".
Revelations that MI6 failed to raise the alarm about his disappearance for more than a week prompted elaborate conspiracy theories about his job and private life.
Several years before his death, Mr Williams tied himself to his bed and had to be cut free by his landlord and landlady.
It also emerged that the bachelor stored STG20,000 ($A31,500)-worth of women's clothes in his immaculate flat and was fascinated by drag queens.
But Dr Wilcox said there was no evidence to suggest the spy was a transvestite "or interested in any such thing".
The make-up found in his flat was more likely to reflect his interest in fashion, she argued.
And the wigs that were found there were "far more consistent with dress-up such as attendance at a manga conference", she added.
There was some suggestion that his interest in female footwear could have been of a sexual nature, but this was not unusual, Dr Wilcox observed.
"Gareth was naked in a bag when he was found, not cross-dressed, not in high-heeled shoes," she said.
Mr Williams would have been unlikely to invite a third party who was not a family member into his home, Dr Wilcox said, adding: "If a third party was present at the time of his death, in my view that third party would have to have been someone he knew or someone who was there without an invitation."
If someone else was there without having been invited, "that raises the possibility of an illegitimate purpose on the third party's part", she said.
"Gareth may have worked out a technique how to get into the bag and lock it from inside, but I find it extremely unlikely that he did so."
Detective Chief Inspector Jackie Sebire, who is leading the investigation, said the inquest had raised "several new lines of inquiry and the investigation will now refocus and actively pursue all the evidence heard and all the new lines of inquiry".