Twenty years ago this month, Princess Diana was killed in a car crash in Paris’ Pont de L’Alma tunnel.

Her death was the subject of an exhaustive inquiry in the British courts, which ultimately concluded that Diana’s “unlawful death” was caused by the reckless driving of limousine driver Henri Paul and the paparazzi pursuing Diana and Dodi Fayed through the streets of Paris.

The inquest heard that the driver, Henri Paul, was driving at an excessive speed and had five times the legal limit for driving of alcohol in his system.

It was also also suggested that Diana and her boyfriend, Dodi Fayed, may have survived the crash if they had been wearing seatbelts.

Over the years, the findings of the inquiry have been presented as the definitive answer to Diana’s death.

However, the official explanations have notably failed to convince an army of Diana conspiracy theorists who believe that the death was actually engineered by the British establishment, and that the malfeasance was then covered up.

The reasons for the suspicion are not hard to fathom, starting with motive.

Diana’s death, even if unwished for, certainly suited the royal family, and especially Charles. It’s hard to imagine Charles ever having been able to marry his long term lover Camilla Parker Bowles, let alone make her queen, had Diana lived.

Diana herself suggested in a letter that is in the public domain that Charles was planning to kill her.

She wrote: “I am sitting here at my desk today in October, longing for someone to hug me and encourage me to keep strong and hold my head high.

“This particular phase in my life is the most dangerous—my husband is planning ‘an accident’ in my car, brake failure and serious head injury.”

The queen is said to have remarked that somebody might have “greased her brakes” when she first heard of Diana’s crash.

And as recently as three years ago, the Metropolitan police confirmed that it was investigating new evidence of foul play, although ultimately the “scoping” exercise investigating claims by an ex-Special Forces operative who had boasted that the SAS “was behind Princess Diana’s death” in conversations with his ex-wife came to nothing.

One of the most prominent voices to cast doubt on the official version of the events is the journalist and playwright Jon Conway who in his play, Truth, Lies, Diana, dramatized conversations with and evidence given by key witnesses.

One of the most extraordinary facts he attempted to illuminate was the unorthodox way the emergency and medical services handled the incident.

He obtained a transcript of the call made by Dr. Jean-Marc Martino, the emergency doctor at the crash site, who radioed in and said that there were two deaths—Dodi Fayed and Henri Paul—but said specifically that there were “no thoracic injuries.” As Conway points out, that’s exactly what Diana died of.

Another odd fact is that the ambulance did not arrive at the scene until 12:40 a.m., 17 minutes after the first call had been made.

That it then took over an hour for Diana to be freed from the car and loaded into the ambulance is perhaps not utterly extraordinary, but the fact that the ambulance actually stopped for several minutes on its way to the hospital has always made conspiracists suspicious that perhaps there was a concerted effort to deny Diana timely treatment.

Indeed, Martino was asked at the inquest if there was “any truth in the suggestion you deliberately delayed taking her to hospital in order to harm her?”

Unsurprisingly he denied the suggestion, and said they had to stop the ambulance because her blood pressure dropped: “We had to find out why there was such a drop in the blood pressure. We were afraid that would lead to a cardiac arrest. It’s very hard to resuscitate a patient in those condition whilst the ambulance is moving.”

The ambulance eventually arrived at hospital at 2:06 a.m. and despite the efforts of medical staff, Diana died a few hours later.

Conway and other critics, including the late writer and publican Noel Botham, who ran the Soho pub the French House and penned the book The Murder of Princess Diana, argued that medical norms would see one rushing to the hospital—just a few minutes away—as fast as possible rather than trying to administer treatment in the back of an ambulance.

Events that unfolded at the hospital after Diana was pronounced dead were also unorthodox.

According to the writer Christopher Andersen, whose book, The Day Diana Died, has recently been reissued ahead of the anniversary of her death, the princess’ body was frisked for royal jewelry at the queen’s request.

“If there were any royal jewels among Diana’s effects, Her Majesty wanted them returned to the Royal Family immediately,” Andersen writes.

Andersen also relates how Diana was named on her medical chart as Patricia, an extraordinarily incompetent attempt at subterfuge.

Conway, in a conversation with The Daily Beast, pointed to a litany of suspicious discrepancies and circumstances, such as the fact that the driver, Henri Paul, who was said to be five times over the legal alcohol limit for driving, appeared to be acting quite normally in security footage that has been widely disseminated.

“You see him walking through the door, tying his shoelaces,” says Conway. “If he was five times over the limit he would have stank of alcohol, there’s no way anyone would have got in a car with him.”

Henri Paul had close links with France’s security services, and Mohamed Fayed, whose son Dodi was killed in the crash with Diana in August 1997, has long argued the couple were murdered in an MI6-led plot on the orders of the Duke of Edinburgh, who was fearful of the ramifications of Diana marrying a Muslim.

Al Fayed claimed Diana was pregnant with his son’s child and the royal family was horrified by the prospect of a union with a Muslim family.

The former owner of Harrods alleged that the queen’s husband had instructed MI6 to carry out the hit. “Prince Philip is the one responsible for giving the order,” he said. “He is very racist. He is of German blood, and I’m sure he is a Nazi sympathizer.”

Then there is the famous white Fiat Uno, which allegedly clipped Diana’s car seconds before the crash precipitating the disaster.

The driver is said to have been James Andanson, a photographer who followed the princess’ every move in the week before her death.

He was thought to have committed suicide when his burnt corpse was found in the wreckage of a car in the French countryside in 2007—but a fireman who attended the scene said: “I saw him at close range and I’m absolutely convinced that he had been shot in the head, twice.”

Adding fuel to the fire is the fact that no footage has ever been recovered from any of the 14 CCTV cameras in the Pont de l’Alma underpass. None, allegedly, recorded video of the fatal collision, because they were either not working or facing the wrong way.

The only survivor of the crash, Trevor Rees-Jones, has always maintained he has no memory of the events of the night.

At the inquest, Rees-Jones said: “I am not part of a conspiracy to suppress the truth at all. All I have ever done is give the truth as I see it.”

In a bizarre coincidence, when Barry Mannakee, the former police protection officer whom Diana is believed to have had an affair with and was fired as a result, died in a road traffic accident, the only other witness to his death also claimed lack of recall.

Diana suggested in a recorded video tape that Mannakee had been “bumped off” in a staged motorcycle crash.

As with so many conspiracies, it is almost impossible to disprove every theory.

But, 20 years on, it is hard to shake the niggling feeling that there is more to Diana’s death than a drunk chauffeur.

And the truth of one observation will become very clear the day Prince Charles accedes to the throne: that as far as the establishment was concerned, Diana’s death was extremely convenient indeed.