(Tom Morgan) Gareth Williams, 31, dug out the guestlist for an event the former American president was going to as a favour for a pal.
The codebreaker — who had breached his security clearance — handed the list to the friend, who was also to be a guest.
MI6 bosses raged over the data breach amid growing tensions with US security services over Mr Williams’s transatlantic work.
Today, just over five years since his body was found inside a padlocked bag, his death remains one of Britain’s most mysterious unsolved cases.
The Sun on Sunday can reveal that voicemail messages Mr Williams left for family and pals were deleted in the days after his death. And a rival agent may also have broken into the flat to destroy or remove evidence.
The inquest was barred from discussing Mr Williams’s work in public. But sources say he was helping on the joint monitoring network Echelon, which uses sophisticated programs to eavesdrop on terrorists and criminal gangs, particularly those in Russia.
Echelon is used by Britain, the US, Canada, Australia and New Zealand.
A source said: “The Clinton diary hack came at a time when Williams’s work with America was of the most sensitive nature.
“It was a diplomatic nightmare for Sir John Sawers, the new director of MI6 at the time.”
Insiders claim Mr Williams, who had been given a second passport with a fresh identity, was also getting fed up with living a secret life. He is said to have loathed his spy training after having his wrist broken during one hardcore session.
One insider said: “Williams’s state of mind in the months before his death was worrying those closest to him.
“He found the training so stressful and his mood blackened even talking about it.
“Typically he’d be asked to learn a new identity then report to a country hotel to meet an interrogation team. There he would be grilled about his new ID for 48 hours without sleep.
“His wrist was broken once after he was handcuffed to a metal bar inside a van that was driven around the country for several hours while he faced a barrage of questions.”
His sister Ceri Subbe also told the inquest he did not enjoy the “flash car competition and post-work drinking culture” of MI6.
He had applied to return to GCHQ, in Cheltenham, but bosses were slow in approving this.
Mr Williams, a keen cyclist originally from Anglesey, North Wales, died shortly after returning from a hacking conference in America.
He had been to see a drag queen show by himself two days before he was last seen alive, on August 15, 2010.
Eight days later his naked body was found folded into the 32in by 19in bag placed in the bath of his flat in Pimlico, central London.
His mobile phone and sim cards were laid out on a table. The last computer evidence of him being alive showed him looking at a cycling website.
Detectives are still baffled as to how the maths genius and expert cryptographer died.
An initial line of inquiry was that he was killed by a jealous lover. Yet there were no signs of forced entry to the flat.
In 2012, lawyers for his family said he could have been killed by someone who specialised in the “dark arts of the secret services”. The police did not rule out his intelligence work playing a part in his death.
They thought he may have been stuffed in the bag by killers who later broke back in to cover their tracks.
Investigators also suspect the flat had been “steam-cleaned”, which would explain why no DNA evidence was found.
The nature of Mr Williams’s work remains a secret, but sources claim he dealt with equipment that tracked the flow of cash from Russia to Europe. The technology let MI6 follow money trails from accounts in Russia to criminal gangs.
A Kremlin car was spotted near his home on the day he was last seen alive.
Police also issued e-fits of a “Mediterranean” couple said to have visited Mr Williams in either June or July.
Coroner Dr Fiona Wilcox, who heard the 2012 inquest into his death, criticised MI6 for failing to report Mr Williams missing for a week. The delay meant a Home Office pathologist was unable to find a cause of death.
Dr Wilcox concluded that Mr Williams’s death was “unnatural and likely to have been criminally mediated”.
She ruled out his interest in bondage and drag queens as having any bearing, adding: “I wonder if this was an attempt by some third party to manipulate the evidence.”
She also dismissed speculation that Mr Williams died due to some kind of “auto-erotic activity” and denied he had any interest in claustrophilia, the love of enclosed spaces.
Experts said even escapologist Harry Houdini would have struggled to lock himself in the bag. Pathologists said Mr Williams would have suffocated within three minutes if he was still alive when put in there.
Yet a year later, Scotland Yard ended a review of the investigation, saying it was more likely Mr Williams had locked himself in the bag and no one else was involved. The announcement angered Mr Williams’s family, who said they stood by the coroner’s findings.
Last night a Met spokesman said: “The death of Gareth Williams was subject to a thorough investigation and coroner’s inquest. We are not prepared to speculate.”