It has been a mystery that has fascinated the world for the past 22 years, spawning dozens of conspiracy theories, leaving a dark shadow over the royal family and forever tarnishing unfaithful Prince Charles.
Princess Diana herself predicted that she would die in a car accident orchestrated by her ex-husband after their toxic marriage disintegrated because of his obsessive love for his mistress Camilla, who he married after Diana's death.
But new evidence proves beyond any doubt what really happened in that Paris tunnel on the night of August 31, 1997, when the People's Princess and her Egyptian lover Dodi Fayed died after their Mercedes sedan smashed into a pillar at high speed.
The explosive findings contained in a new book, Diana: Case Solved, may lead to a new enquiry into her tragic death and an investigation as to why French and British authorities – together with the royal family – went to such lengths to cover up the circumstances surrounding her death.
"We interviewed new witnesses who saw the crash and its immediate aftermath," explains journalist Dylan Howard, who co-wrote the book with respected former Victorian homicide detective Colin McLaren.
"And crucially unearth the one man who knows for sure what happened – and who for 22 years has been ordered to remain silent. He was the keeper of the world's biggest secret of all. From the outset, the French police made astonishing errors and misjudgements."
The new witness is French-Vietnamese national Le Van Thanh, who claimed he was working security at the time of the crash, but was actually seen leaving the scene in his white Fiat Uno, which was badly damaged when it collided with Diana's Mercedes.
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Le Van, 54, was arrested in November 1997 and questioned for six hours. He was released after French police claimed he had an alibi, but earlier this year the father-of-two broke his 22-year silence to change his story – completely obliterating the official version of what happened that night.
French authorities have always maintained the crash happened because driver Henri Paul was drunk and lost control of the Mercedes after he was blinded by the flash of a camera from one of the pack of paparazzi photographers chasing Diana and Dodi.
But the new evidence, backed up by forensic and crime scene experts, along with the testimony from witnesses the police failed to find at the time, show that this version is completely wrong and part of an official cover-up.
Colin says the new evidence he has gathered proves that Le Van's car clipped Diana's Mercedes as he slowly entered the tunnel from a merging lane, after Henri braked suddenly – because of the flash of a police speed camera.
"A fragment of plastic found by police came from a Fiat Uno and Diana's Mercedes was found to have a smear of white paint on it," he says, adding that Le Van confirmed his brother painted his damaged white Fiat red just days after the accident.
"An exhaustive database search of over 12,000 Fiats never found the elusive car. Despite calls to the public, no person ever came forward as the driver."
Colin and Dylan do not completely discount the theory that a mysterious lone motorcyclist, who has never been found but was seen in the tunnel at the time, could also have contributed to the shocking series of events in the tunnel that night.
The experts, who took a fresh look at all the old evidence together with their new discoveries, claim that Henri was travelling at around 190km/h – a much higher speed than previously thought – which caused the Mercedes to skid before it became airborne and smashed into the pillar.
"The Mercedes S280 sedan carrying the world's most famous woman could have only become airborne after hitting a second car and leaving the road surface – and staying airborne for 85 feet [26 metres]," says Colin. "The Mercedes had clearly reached an enormous speed on its approach path."
Colin has little doubt that the driver of the white Fiat Uno that police tried to find at the time of the tragedy is Le Van Than, who admits that when he considered changing his story, he was warned that he could be jailed.
"His own sister stepped forward and told him not to sign the fresh statement, as he could be prosecuted and sent to jail for five years as he 'refused to stop and help injured people at the scene of an accident'," they claim.
Their major investigation concludes that while many important people and organisations had motives to kill Diana, Le Van was not part of a plot and that her death was caused by a "simple and routine car accident" and the fact she wasn't wearing a seatbelt.
Drunk and estimated to be driving at more than double the speed limit, Henri Paul – the driver of the Mercedes S280 that was taking Diana and her lover Dodi back to their Paris apartment – has long been held accountable for her death by French and British authorities.
But mystery has long surrounded the involvement of Henri, who also lost his life in the accident, and just why he was the one chauffeuring the couple that evening.
In Diana: Case Solved, the author reveals there were a number of suspicious circumstances surrounding Henri, who at the time was the deputy head of security at Paris' Ritz hotel, which is owned by Dodi's father Mohamed Al-Fayed.
The author writes, "Henri Paul was a 41-year-old professional driver. Yet when he died, it was revealed that he had the equivalent of approximately $340,000 [$A495,000] in multiple bank accounts. How did this happen, and who was paying him? Many alleged that Paul had earned this money by working covertly for security services. By spying. But for what or whom, it remains to be seen.
"Henri Paul was responsible for the safe travel of the most photographed, most famous woman in the world. Yet his blood was a mess of alcohol, Prozac and anti-alcohol pharmaceuticals.
"How did Paul come to be driving Diana that night? Is drunken driving the chief culprit in explaining Diana's accident, or is something more sinister at play?"
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For years he kept quiet as the memory of a dying Diana haunted him every day. For French firefighter Xavier Gourmelon, 51, who was one of the first emergency workers to reach the princess immediately after the car accident that took her life, it's a scene that will forever be ingrained in his mind.
In Diana: Case Solved, it's revealed Xavier did all he could to save the People's Princess, who was mysteriously free from blood and clinging onto consciousness when Xavier reached her and began administering CPR.
"She was moving very slightly and I could see she was alive," Xavier recalls. "I held her hand and told her to be calm and keep still. I told her I was there to help and reassured her.
"I massaged her heart and a few seconds later she started breathing again. I expected her to live. But I found out later that she had died in hospital.
"The whole episode was very much on my mind. And the memory of that night will stay with me forever."
Diana: Case Solved: The Definitive Account That Proves What Really Happened by Dylan Howard with Colin McLaren (Skyhorse Publishing) is out September 17. You can buy it here