Diana died as a result of a car crash on the night Aug. 31, 1997, as she was being driven through the streets of Paris. Her companion, Dodi Fayed, also perished in the crash.
Fayed's father, Mohamed Al Fayed, claims the couple were targets of a plot directed by Prince Philip, husband of Queen Elizabeth II, and carried out by British intelligence agents. Conspiracy believers have also claimed that their deaths were engineered to head off a reported engagement, or a possible pregnancy.
French and British police blamed the crash on Henri Paul, acting head of security at the Ritz Hotel, who was driving the couple's car. Al Fayed disputes police conclusions that Paul was drunk.
At the start of the ongoing inquest at London's High Court, Al Fayed bluntly told reporters, "My son and Princess Diana were murdered by the royal family." He says, publicly and often, as CBS News correspondent Sheila MacVicar puts it, that Philip organized their assassination.
Philip, notes MacVicar, is said to have sometimes been critical of the princess, and is alleged to have written her brutal letters. There has been testimony that Diana herself claimed to have had a warm relationship with both the prince and the Queen.
"It could be the nearest they get to Philip, and you don't want to let him go without asking all the difficult questions, do you?" observed Martyn Gregory, author of "Diana: The Last Days."
Attorneys will argue that the prince himself should appear to give evidence, but Gregory told MacVicar, "I would be astonished if he were to order Prince Philip to appear. I think the chances are somewhere between nil and zero."
The basis for the conspiracies, MacVicar explains, are claims by al Fayed that Diana was pregnant, and about to marry Dodi -- and that the British establishment, led by Philip, couldn't accept Dodi, an Egyptian Muslim, as stepfather to Diana's son and future king, William.
On Wednesday, Diana's stepmother told the court she believed the princess and Dodi were 'blissfully happy together" and would have married.
But Raine, Countess Spencer also said Diana was "far too old-fashioned to get pregnant outside of marriage." She added, "It is very discourteous to both Diana and to Dodi and to everyone concerned that it should be written off, their relationship, as a summer romance. It was much deeper, much more profound and much closer."
Also Thursday, one of Diana's closest friends is scheduled to testify. MacVicar says the friend has already said she knows Diana could not have been pregnant, because she was with her ten days earlier, when Diana had her period -- and that the princess was using birth control.