By Nicholas Witchell
BBC royal correspondent
The inquest into the deaths of Diana, Princess of Wales, and her companion, Dodi Al Fayed, has ruled they were unlawfully killed by the grossly negligent driving of both their chauffeur and of the following paparazzi.
Mohamed Al Fayed has always believed his son was murdered
The collapse, when it came, was astonishing.
For more than 10 years Mohamed Al Fayed has been utterly convinced the events in the Pont de l'Alma tunnel in Paris were deliberately engineered by agents of the British state whose purpose was to prevent Diana, Princess of Wales, from marrying a Muslim, his son Dodi.
No-one who heard Mohamed Al Fayed give evidence to the inquests into the couple's deaths could fail to have been struck by the intensity and complexity of this bereaved man's feelings.
And that, perhaps, has been the fundamental problem. Mohamed Al Fayed has never been able to step back dispassionately and consider the evidence about what happened that night.
He made up his mind the moment he was telephoned by the president of the Ritz Hotel to be told that the accident had occurred and that Dodi was dead and Diana injured.
"This is not an accident. It is a plot, an assassination." Those were Mr Al Fayed's words in the early hours of the morning of 31 August 1997.
He has never wavered from that belief. The only modification he has made has been in the scale of the plot that he has alleged.
List of conspirators
From a very early stage he claimed the conspiracy had been instigated by Diana's former father-in-law, the Duke of Edinburgh.
But by the time Mr Al Fayed left the witness box in Court 73 at the Royal Courts of Justice on 18 February, the list of supposed conspirators had reached astonishing proportions - so astonishing, really, that the "conspiracy" appeared to have lost any connection with reality.
The cost of inquiries into Princess Diana's death is almost £7m
By the end of that day Mr Al Fayed had named the following as conspirators: the princess's former husband, the Prince of Wales; the former British Prime Minister, Tony Blair; two former commissioners of the Metropolitan Police, Paul Condon and John Stevens; the Queen's former private secretary Lord Fellowes and the former British ambassador to Paris, Lord Jay.
But that was not all. According to Mr Al Fayed, Diana's elder sister Lady Sarah McCorquodale had been part of the cover-up. The Al Fayed bodyguard Trevor Rees who was severely injured in the crash had been described as a "crook" who had also, according to Mr Al Fayed, been part of the conspiracy.
Similar accusations had been levelled against the intelligence services of Britain, France and the United States, together with the French emergency, medical and judicial authorities.
All - according to Mohamed Al Fayed - had been part of the plot to carry out, or cover up, multiple murder.
The major difficulty for Mr Al Fayed and his many advisers and lawyers is that there is not, and indeed never has been, a shred of what could properly be described as evidence to support the central proposition that the deaths were the result of a plot, rather than the very much more prosaic alternative that the couple died in a tragic accident, caused by Henri Paul drinking too much and driving too fast as he tried to out-run the reckless pursuit of the Paris paparazzi.
It became more and more apparent that... there simply was not anything of substance to support the Al Fayed claims
For 10 years, Mr Al Fayed's obsessive belief in a murder plot has been encouraged by the uncritical efforts of his staff and the cynical attention of certain newspaper proprietors who know that any story about a Diana "conspiracy" is good for circulation.
Together they have kept the story firmly in the public arena and encouraged many millions of people to believe that there must be more to the conspiracy claims than those claims have ever, in truth, deserved.
But beliefs, however obsessive, simply are not sufficient when matters move into a courtroom.
What counts is evidence, and as the months of these particular inquests progressed it became more and more apparent that, for all the skill of the three high-powered legal teams representing, variously, Mr Al Fayed himself, the parents of Mr Paul and the Ritz Hotel, there simply was not anything of substance to support the Al Fayed claims.
It is true that the French made a mess of the first post-mortem examination carried out on the body of Mr Paul on the morning of Sunday 31 August.
Samples were poorly labelled, leading one eminent British toxicologist to tell the inquests that he had never encountered such a "disgraceful forensic trail" in the 40 years of his career.
But the French authorities realised the first set of blood and tissue samples were open to question, and went back three days later and took a second set in properly controlled and carefully supervised circumstances.
What emerged very clearly from the inquests was that, irrespective of any questions that may properly be raised by the bungled first post-mortem examination, the fact is that Mr Paul was, without question, under the influence of alcohol when he took the wheel of the Mercedes.
One witness said he had seen Mr Paul in the Bar Burgoyne near his home at about 2020 local time on the evening of 30 August.
And, most importantly, when Mr Paul returned to the Ritz at 2200 local time to help deal with the unexpected return to the hotel of Dodi and Diana, he went to the hotel's Bar Vendome where he drank two double measures of Ricard.
The inquests heard that Ricard is a more potent spirit, measure for measure, than most others. Expert evidence was heard that in the Bar Vendome alone, Mr Paul consumed the equivalent of four single whiskies.
As one of Britain's leading forensic toxicologists, Professor Robert Forrest, said: "Henri Paul was not fit to drive when he got into that Mercedes."
Then there was the compelling evidence of witness after witness who saw the Mercedes approaching the Pont de l'Alma tunnel at considerable speed.
This is what some of them said: "The Mercedes overtook me going very, very fast... you don't drive at that speed on the embankment on a Saturday night... it was travelling at an inordinate speed... the driver was mad to drive that fast."
Liars, fantasists and embellishers
In contrast to the obvious honesty and authenticity of the majority of the nearly 250 witnesses, was the palpable unreliability and even dishonesty of a small group.
Unfortunately for Mr Al Fayed, the witnesses who spoke of a "blinding flash" in the tunnel and of sinister "men in black" on a motorcycle came, very definitely, in the latter category.
In short, the fantasies and the fantasists were finally found out and revealed for what they are
Yes, the inquests revealed the liars, the fantasists and the embellishers.
As the coroner, Lord Justice Scott Baker, said in his summing up: "A number of people seem to have had a compelling desire to pretend they were there when in truth they were not there at all."
People who have evidently dined out for the past 10 years by telling enhanced or invented stories of where they were or what they saw that night found themselves being cross-examined by five leading British barristers.
In short, the fantasies and the fantasists were finally found out and revealed for what they are.
Threw in the towel
Sadly, and without wishing to sound unsympathetic to a man who suffered a grievous loss, that brings us back to Mohamed Al Fayed himself, and to the collapse of his case.
After nearly 90 days of exhaustive inquiry by the inquests, involving evidence from nearly 250 witnesses, the Al Fayed legal teams threw in the towel.
They accepted there was not a shred of evidence to support properly the central pillars of the conspiracy theory.
It happened without warning during the final week of evidence, during submissions when the jury was not present - which meant, of course, that it could not be reported at the time.
First, Al Fayed lawyers accepted that there was no direct evidence that either the Duke of Edinburgh or the secret intelligence service, MI6, had been involved in any murder conspiracy involving the princess or Dodi Al Fayed.
Equally without warning, a few days later in the course of further submissions without the jury, Michael Mansfield, the senior Al Fayed barrister, turned to the role of the white Fiat Uno - the untraced car that was struck a glancing blow by the Mercedes as it entered the Pont de l'Alma tunnel and which, for the past decade, has been one of the principal components of the supposed "conspiracy".
On Thursday 20 March, Mr Mansfield told the court: "The Fiat Uno was not the cause of any loss of control [by the Mercedes]. We are not suggesting its driver is guilty of anything."
By any measure, this was an astonishing admission. The vehicle and driver that had launched 1,001 conspiracy stories was suddenly in the clear and not implicated in anything sinister.
But more was to follow. In a closing written submission to the coroner, the Al Fayed lawyers accepted that there was no evidence to support the assertion that Diana was illegally embalmed in order to cover up a pregnancy - a "pregnancy" moreover which, they accepted, could not be established by any medical evidence.
And finally they accepted that there was no evidence to support the assertion the French emergency and medical services had played any role in a conspiracy to harm the princess.
In the uncharacteristically incautious words of lawyers for the Metropolitan Police, this was "a collapse of monumental proportions".
By the time the jury was sent out to consider its verdicts there was, in reality, nothing left to the conspiracy theories.
The edifice brought together at enormous cost and with undoubted sincerity by Mohamed Al Fayed during the past decade had been revealed to have no foundations.
It has taken six months and almost £7m of British taxpayers' money. That is the estimated cost of the inquests and the inquiry by the Metropolitan Police that preceded it.
But, finally, the conspiracy theories have been run to ground and shown for what they are - a fantasy borne of one man's anguished denial of the truth, and of the need of many to believe that a beloved princess must have been the victim of something more elaborate and malevolent than a reckless aberration by a single motorist.