The words 'white Fiat Uno' could join 'grassy knoll' and 'Roswell, New Mexico' as universal shorthand for conspiracy and cover-up.

Soon after the accident, rumours and reports were heard of a white car zigzagging in front of the Mercedes in which Diana, Princess of Wales, and Dodi Fayed died, moments before the fatal crash.

The car has not been found, but that does not worry conspiracy theorists. They thrive on uncertainty. To them, even accepted facts appear as part of a grand cover-up.

The wreckage which still inspires new explanations

Questions about the deaths abound in their minds, such as:

• What did Diana mean when she said, a few days before her death, that she would shortly do something that would shock everyone?
• How could it have taken nearly half an hour for the Mercedes to reach the crash site when the tunnel is only a couple of miles from the Ritz Hotel, especially as the car was travelling at high speed.
• Is it true that Paris was swarming with secret service agents and special forces during that fateful weekend?
• Wouldn't a Mercedes limousine have protected its occupants better from a crash?
• Did Diana actually manage to speak some last words while she lay dying in the hospital?
• Were there traces of drugs in Dodi's and Diana's blood?
• Isn't it strange that, after all the money spent on the investigation, the white Fiat has not been located?

But many theories go much more to the heart of the matter. Conspiracy theorists don't just ask questions, they provide answers too.

A new life?

A common notion is that the deaths were staged to enable Diana and Dodi to start a new life away from the world's prying eyes. This basic theory is embroidered by ideas that Dodi's experience at film production helped stage manage the event.

Security forces colluded to ensure that the press did not find out what was going on, the theories run. This would leave the couple free to marry and live together, probably on a private island, never to be disturbed again.

Some even suggest that with plastic surgery, the princess could return to look after her sons. "Look out for a similarly built nanny coming on the scene in the near future," one theorist writes.

Civil war

Some conspiracists speculate on who would have anything to gain from the deaths. The usual suspects are the Royal Family and the British Government (although the Israeli secret service, Mossad, is suspected by some).

Reasons include supposed hatred for Diana, resistance to the idea that the mother of the heir to the throne could be involved with a Muslim, and freeing Prince Charles to marry his friend Camilla Parker-Bowles. One particularly imaginative 'motive' includes wanting to avert a civil war (between Prince William and any future half-brother).

Dodi Fayed

Profit mongers

Another group which comes under question is the manufacturers of landmines, who found their business interests under attack from Diana's campaign against their products.

Or perhaps it was a business rival of Dodi Fayed's father, Mohammed Al Fayed. Some have even alleged that the British Conservative Party was behind the crash, in revenge for the dispute over the Cash-for-Questions affair in which Mr Al Fayed gave money to former Tory MP Neil Hamilton.

Yet most of these theories are advanced only by individuals who are doing a bit of kite-flying.

'There was a conspiracy'

Mr Al Fayed himself, however, has been the most prominent character in the drama to say that he believes his son and Diana were not killed in an accident.

Under the headline 'It was no accident', he told the Mirror newspaper earlier this month: "I believe there were people who did not want Dodi and Diana to be together."

"There was a conspiracy and I will not rest until I have established exactly what happened."

Whatever the conclusion of the investigation - but especially if it backs the conventional accident explanation - it seems the conspiracists will not be alone in suspecting foul play.

BBC News special report online:
Death of a Princess: Six months on...

• The grief of millions
• On the Net: Truly the People's Princess
• "Why did Diana die?"
• The battle for Diana's lucrative legacy
• the changing face of the monarchy