The genus of most news investigations can be narrowed down: a whistleblower or aggrieved former employee, a paid source, or a journalist with the belief that there is a story to be uncovered.
In the case of the BBC's "Reinventing the Royals", which has been pulled mysteriously from Sunday's schedule at the eleventh hour, a paid source can probably be ruled out. The BBC has strict rules on payments, especially if they undermine the standing of the Royal family and the documentary could certainly do that with its central allegation that Mark Bolland, Prince Charles' spin doctor after the death of Diana, Princess of Wales, schemed and contrived to manipulate the press to raise the profile of his boss, even by cynically using Princes William and Harry.
A clue could be in the interview with Sandy Henney, Charles' press secretary when the Princess died in 1997. She was very close to the boys, very much a mother figure after the tragic accident in Paris and was said to have been manipulated out of her job by Bolland. It is her first ever public interview.
Another lead is Steve Hewlett, the presenter and director of the two-part documentary, who was editor of Panorama when Diana gave the programme an astonishing interview in which she outed Camilla Parker Bowles with the memorable phrase "there were three of us in this marriage so it was a bit crowded".
It is also the 20th anniversary of the 1995 interview that Hewlett has described as the "scoop of the century" so he has good reason to return to his finest hour as editor of Panorama. Maybe the inspiration for the programme is a mixture of Hewlett and Henney. The more difficult question is why has it been pulled, seemingly on the orders of James Harding the BBC news supremo?
Hewlett is no mug; a talented and experienced journalist who knows the workings of the BBC inside out, who would recognise that the corporation's lawyers had to be involved from the very outset and work with them to deliver a programme that would not end up in the libel courts.
The BBC, as well as employing lawyers who are wise in the ways of defamation, have a raft of legal brains whose sole job is to wind their way around issues of copyright. One of the reasons for pulling the documentary is said by the BBC to be copyright issues over archive footage which have been raised by Prince Charles' legal team. There may be a sliver of truth in that statement, but not much.
The documentary would not have been sent to the Palace of Westminster for comment unless the BBC was sure that it was factually correct, would be balanced and be legally safe - but how much did they know about the issue of Henney's confidentiality and how much of a chance did they take on the Royal lawyers not imposing it?
The days of the Royal family neither "complaining or explaining" are long gone and while it turns a blind eye to former employees who break confidentiality in praising the Royal firm, Henney's take would be a different matter.
The documentary also includes an interview with Tom Bradby, the ITV journalist know to be close to Prince William, who would not have spoken without contacting his friend. Was it a punt by the programme makers in hoping Charles would take no action against a documentary that included his son's chum?
If Henney was taken to court it would be a messy case and be politically embarrassing for the BBC, the last thing it wants with a new chair of trustees and a fight to retain the licence fee.