In August 2010 the death of a brilliant young mathematician in extremely bizarre circumstances sent shockwaves around the world.
The badly decomposed body of Gareth Williams, 31, was discovered by police locked inside an airtight holdall in the bath of his central London flat.
The revelation that Mr Williams was a secret agent working for MI6 catapulted the story on to the front pages and spawned countless conspiracy theories as to how he might have died. Speculation ranged from a state-sponsored assassination to a sex game gone wrong.
But despite a coroner’s inquest and two lengthy police investigations, his grieving family and the wider world remain no closer to understanding exactly what happened to Mr Williams.
Now, almost a decade on from his untimely death, the Telegraph has re-examined the leading theories and spoken to a number of expert sources in order to assess whether the truth will ever be revealed.
Gareth Williams was born and brought up in a small Welsh-speaking community in Anglesey, North Wales.
A child prodigy he took his maths GCSE while still at primary school, completed his A-level by the age of 13 and graduated from Bangor University at 17.
While conducting postgraduate research at Cambridge University he was approached by scouts from GCHQ and offered a role working as an analyst at its secretive Cheltenham headquarters.
In 2008 he was seconded to the Secret Intelligence Service, or MI6, based at Vauxhall in London, where he successfully completed training for operational deployment.
On the afternoon of Monday Aug 23, officers from the Metropolitan Police attend Mr Williams’s top floor flat at 36 Alderney Street in Pimlico amid concern for his welfare.
His body was found in a large, red, North Face holdall, which had been zipped and padlocked from the outside.
Despite being almost 70 degrees Fahrenheit outside, the heating in the flat had been turned up to its highest setting.
The flat was neat and undisturbed and it later emerged the property had been 'cleaned' to such an extent that virtually all traces of fingerprints and DNA had been erased.
The police investigation
The investigation was initially handled by Scotland Yard's murder squad, but due to the sensitive nature of Mr Williams's work, overall responsibility was quickly handed to the counter terrorism command, whose officers have a higher security clearance than most of their Metropolitan Police colleagues.
An Executive Liaison Group was established with representatives of MI6, MI5 and Scotland Yard discussing how to progress the investigation while ensuring they did not interfere with matters of national security.
While Scotland Yard retained the overall lead on the investigation, MI6 vetoed its detectives having access to Mr Williams’s desk or computers and some members of the SIS also declined to be interviewed.
Killed by a foreign state? The evidence for...
Mr Williams’s role as an MI6 agent led to immediate speculation that he had been assassinated by a hostile foreign state, with Russia thought by many to be the most likely culprit.
The fact that his flat appeared to have been professionally ‘scrubbed’ of DNA and fingerprints further added weight to this theory.
Philip Ingram, a former intelligence officer, said the murder of Alexander Litvinenko in London in 2006 and the attempted assassination of Sergei Skripal and his daughter Yulia in Salisbury last year, proved the Russians were capable and willing to act on UK soil.
Mr Ingram said: “Recent history has shown us that certain rogue countries are willing to resort to murder for all sorts of reasons. But if this was a state-sponsored killing, then the method suggests it was more about sending a message, than about simply getting rid of someone.
“Such a murder would be an effective message to other agents operating elsewhere, 'continue to cooperate, or this could happen to you'.
“Or it could have served as a message to their own people who might be thinking of defecting, ‘beware, this could happen to you’.
“I firmly believe that the attack on the Skripals was never just about trying to kill Sergei, it was about sending a message to Putin's dissenters, that whoever you are, wherever you are, we can still get you, any time, any place, in a way that you will never know.
"That is why they used Novichok which of course they knew would be traced back to Russia.”
But why do it in such a bizarre way..?
In the cases of Alexander Litvinenko and the Skripals, those responsible did little to disguise or hide their role.
But almost a decade on from the death of Mr Williams, nobody seems any wiser as to who was responsible, perhaps suggesting this was not a state-sponsored assassination.
According to an MI6 colleague of Mr Williams, who gave evidence at his inquest, his role within the Secret Intelligence Service rendered him “low risk”.
Despite being cleared for operational deployment, one former intelligence agent told The Telegraph he was most likely working as a liaison officer, ensuring GCHQ and MI6 could operate smoothly alongside one another – and far removed from the James Bond image that the public has of secret agents.
“Gareth’s role is likely to have been focused on helping to link up GCHQ with the SIS and allowing the two agencies to talk to one another and understand complex elements of the work each was doing.
“This sort of role would not have made him an obvious target for such a high-profile hit. Despite what we see in the movies, intelligence agents do not go around killing each other and then covering it all up.”
Mr Ingram said despite the attack on Litvinenko and the Skripals, he is not convinced that a foreign power was behind Mr Williams’s death.
“The suggestion that Gareth was murdered to order by the Kremlin, or perhaps one of the other states that employs black ops around the world, such as North Korea or China, is possible, but I would put it in the ‘unlikely’ bracket. There are none of the hallmarks that we saw in the other high-profile cases."
Former Scotland Yard counter terrorist officer David Videcette is also sceptical that Mr Williams's death was a state-coordinated murder.
“If Gareth had become a target in some way then it is conceivable that a foreign power might order his murder – and of course that will have been investigated.
“But the thing that remains troubling about that theory is ‘why do it in such a bizarre way?’
“Why not just walk up to his front door and shoot him in the face? Why go to all this trouble to lock him inside a bag and leave him there until someone finds him? What was that purpose?
“I find the idea that there is a foreign government that wanted to say ‘we can kill you, put you in a bag and get away with it’ very unlikely and we simply don’t have the evidence to support that theory, so it is just supposition. To arrive at that conclusion you have to ignore a lot of other things along the way.”
Taken out by a powerful crime gang?
While little is known about the top secret work Mr Williams was doing for GCHQ and MI6, some sources have suggested he was helping to develop cyber security defences to protect the global banking sector from attack.
If this was the case it could have made him a target for organised criminals desperate to disrupt his activities.
In the weeks before his death Mr Williams visited a ‘hackers’ conference in Las Vegas, where criminals were known to attend.
Following his death police also appealed for help to trace a mysterious Mediterranean couple who were seen entering his building the previous month.
A counter intelligence source said: “It is possible that something Gareth was doing at work put him in the crosshairs of a powerful organised crime gang, with the resources and the audacity to murder him. They would certainly want to cover their tracks which might explain the lack of clues.”
Mr Ingram said it was possible the unusual circumstances of Mr Williams's death may have been intended as a coded message to others in the criminal underworld.
“Organised crime gangs are usually quite straightforward in their methods but they also have a tradition in some quarters of using symbolism to get their message across. Perhaps the bag was part of that.”
Another theory that has been advanced is that those responsible had been intending to remove Mr Williams’s body from the flat but had aborted the plan at the last minute.
Mr Videcette said: “There has been an assumption that the bag was an integral part of the murder, but perhaps it was simply a way to get him out of the flat unseen? These are the sort of considerations that investigators would have been grappling with.”
'No crime group in the world is that sophisticated'
The nature of Mr Williams’s work both at GCHQ and latterly at the Vauxhall headquarters of MI6 was so top secret that even the Metropolitan Police counter-terror officers investigating deaths were denied access to some areas.
So the idea that an organised crime gang would be aware of what he was up to, even if it did impact on their activities, is highly fanciful, according to experts.
Mr Williams had also undergone extensive training to ensure that he remained safe and secure.
Mr Ingram said: “It would be incredibly difficult for any organised crime gang to discover what someone working at this level was involved in. People in these organisations, particularly those from GCHQ and SIS, do not talk about what they do even with their closest family members.”
Another security source added: “If an agent had had any suspicions that he was being targeted by a criminal network he would have been trained to report his concerns up the chain of command. There are very few, if any, crime groups anywhere in the world sophisticated enough to have pulled this off.”
The fact that those responsible have never been identified also undermines the theory that gangsters were behind it, according to experts.
Mr Ingram said: “MI5, MI6, GCHQ and Scotland Yard's counter terrorism officers would have all been working flat out on this. You cannot get a more detailed level of investigation. They have access to the sorts of things that normal police and investigators can only dream of.
“If you look at the Skripal investigation, it is that sort of team that would have been deployed – and look how quickly they managed to identify those responsible. If they haven't managed to figure it out then it had to have been a very, very sophisticated operation, and there is no organised crime group in the world that is that sophisticated.”
Sex game gone wrong?
Within days of Mr Williams’s body being found it was being reported that he had visited bondage websites, leading to speculation that his death could have been the result of a sex game gone wrong.
It also emerged during the inquest that when he had been living in Cheltenham, his landlady had once found him tied to his own bed and unable to free himself.
He told her he had been testing himself to see if he could get free, but she said she believed it had been sexual rather than an exercise in escapology.
When Mr Williams’s flat was searched, police found around £20,000 worth of designer women’s clothing as well as wigs, make-up and 26 pairs of shoes.
Police said there was no evidence that he was involved in a sexual relationship with anyone at the time of his death.
The theory favoured by some therefore was that Mr Williams harboured a fetish for being enclosed in small enclosed spaces and had climbed into the bag voluntarily by himself in order to gain some sexual gratification.
Pathologist Ian Calder estimated that once the bag was sealed Mr Williams could have succumbed to carbon dioxide toxicity within two to three minutes.
His flat showed no sign of forced entry, lending weight to the theory that Mr Williams was alone when he died.
In 2013 the Metropolitan Police conducted a fresh review of the case and concluded that the most likely explanation was that Mr Williams had killed himself accidentally after getting into the bag.
Mr Ingram said: “There are certainly some unanswered questions around this hypothesis but in general the simplest theories are usually the most likely. There is very little solid evidence of foul play and despite what we see in films and on TV the reality is that the intelligence agencies don't have the power to cover up crimes like murder."
Mr Videcette agrees that Scotland Yard's conclusion that Mr Williams's death was accidental remains the most plausible answer to the mystery.
A plot to divert the press?
One of the most enduring questions surrounding the death of Mr Williams is whether or not he could have climbed into the bag by himself and then sealed and padlocked it.
During the inquest, two experts tried more than 400 times to complete the task, but were unsuccessful.
Troubling questions also remain about the fact the heating was on full blast and the flat had been cleaned of any DNA and fingerprints.
Analysis of Mr Williams’s computers showed he had visited bondage websites, but there was no sign he had ever shown any interest in claustrophilia - the sexual fetish for being in confined spaces.
Sian Lloyd Jones, Mr Williams’s best friend, has dismissed the idea that the stash of female clothing suggested he had a hidden interest in transvestism.
She insisted that the items would have been intended as generous gifts for his female friends.
Another friend, Elizabeth Guthrie, said the wigs and make-up found in the flat were probably costumes for a fancy dress party the pair were planning to attend.
One well-placed security source expressed unease at the way details of Mr Williams’s private life were leaked to the press.
The source said: “Just days after Gareth’s body was found and when his family were still coming to terms with the tragedy, ‘Whitehall sources’ were being quoted alongside stories about the sort of websites he had been visiting.
“It suggests somebody very high up was trying to ensure the direction of travel on this story. Whether that was to deflect journalists from the truth or ensure they understood the real picture is something we can only speculate on.”
Mr Ingram said Mr Williams would have undergone extensive vetting regularly throughout his career in order to expose any areas of his private life that could expose him to risk.
He explained: “As Gareth moved through various departments and roles he would have undergone regular vetting, probably as much as twice a year. They would have looked into everything that might be relevant, friends, family interests, even sexual interests and fetishes.
“Having an interest in certain unusual things is not a bar to serving as long as you declare it. It only becomes a problem when you cover something up. As far as we know nothing of this nature was ever uncovered by his employers.”
DNA traces: who else was involved?
The theory that someone else was involved in the death of Mr Williams has long been favoured by those most familiar with the case.
Dr Fiona Wilcox, the coroner who oversaw the inquest, said in her narrative verdict, it was “highly unlikely” Mr Williams had got into the bag himself and suggested the role of another person was likely to have been “criminally mediated”.
Detective Chief Inspector Jackie Sebire, who led the initial Met investigation, also concluded it was “highly likely that a third party was involved”.
Experts were unable to get into the bag without assistance, and the fact that the key to the padlock was discovered inside the holdall also pointed to the involvement of someone else.
Tiny fragments of DNA belonging to two as yet unidentified people were found on the outside of the holdall, but no other prints or forensic traces were discovered in the flat.
One hypothesis that has been considered is that Mr Williams had got into the bag with the assistance of someone else but then died accidentally.
That person, possibly someone associated with the intelligence community, then panicked and slipped away without alerting the authorities.
But police are yet to find evidence of another person being in the flat at the time Mr Williams died.
Thousands of hours of CCTV footage and other surveillance equipment from the streets around the Pimlico flat have been examined but no likely suspects have ever been identified.
Training for MI6?
Lawyers for Mr Williams's family have claimed that the incident in 2008 in which his landlady found him tied to his bed was part of his training ahead of his first unsuccessful application to join MI6.
This led to speculation that his death may also have been part of a training exercise that went horribly wrong and has since been covered up.
Following the inquest, DCI Sebire said it was possible Mr Williams had "preparing" for courses in "escaping from being tied”, but added that she had not received anything from MI6 to confirm this theory.
However, one intelligence source with an understanding of the training agents undergo said he was sceptical that such reckless exercise would form part of the training, even for an operational MI6 agent.
The source said: “I can't think of a training exercise that would require you to get into an airtight bag. With training exercises there are always protocols in place to protect the safety of individuals that are in training. You do not undertake training by yourself in your own home, this is not how it works.”
Will we ever know what happened?
In 2013 Scotland Yard concluded that Mr Williams probably died accidentally after getting into the bag alone.
His family have rejected that explanation and troubling questions remain about many aspects of the case.
Mr Ingram said: "In our lifetime it is highly unlikely we will get to the bottom of what happened. If there is some Machiavellian explanation it will be sealed away in files for at least 100 years, if not forever. But however appetising a Machiavellian scenario might be, the real explanation is probably the simplest."
Mr Videcette agreed: "I suspect there is only one person who knows the truth and that is the person who locked the bag. That person was probably a confidante of Gareth's. Unless that person decides to come forward and say what happened, we will probably remain in dark, however sometimes you do get deathbed confessions, so perhaps one day we will learn the truth."