The probe was part of Operation Paget, the investigation into the various conspiracy theories surrounding Diana's death launched by the British Metropolitan Police in 2004.
Former head of Scotland Yard John Stevens told the Daily Mail that he spoke to Prince Charles about a note that Princess Diana wrote in 1995 which said, "My husband is planning 'an accident' in my car, brake failure and serious head injury," so that he could marry Tiggy Legge-Bourke, a nanny for Prince William and Prince Harry. Charles and Diana divorced in 1996.
Prince Charles was interviewed at St. James's Palace as a witness two years into the investigation, after the note became public in 2003.
"Yes, allegations had been made about the Prince of Wales and other royals but we had to find or examine the [existing] evidence before we approached him with formal questions," Stevens told the outlet. "We found no other evidence to support the scenario suggested in Diana's note."
"We were left with the note, which in itself was not enough to make Charles a formal suspect," he continued. "If he chose to assist [Operation] Paget, he would be doing so voluntarily as a potential witness. We would not be interviewing him under caution."
Tim Graham Photo Library via Getty Prince Charles and Princess Diana
Stevens reportedly read the note to Prince Charles then asked him: "Why do you think the princess wrote this note, sir?"
The royal replied, "I did not know anything about [the note] until it was published in the media."
"You didn't discuss this note with her, sir?" Stevens asked, to which Charles said, "No, I did not know it existed."
When asked, "Do you know why the princess had these feelings, sir?" Charles replied, "No, I don't."
Georges De Keerle/Getty Princess Diana and Prince Charles
"At the end of the day he was incredibly cooperative because he had nothing to hide," Stevens told the Daily Mail.
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Although Prince Charles, now 72, cooperated with the investigation, his father, Prince Philip, declined to assist in the investigation. He returned a request to comment on the allegations with three words, "No, thank you."
PA Images Princess Diana during her Panorama interview
Princess Diana's note was written around the time she did her famous BBC Panorama interview with Martin Bashir.
An inquiry conducted by former supreme court judge John Dyson found last month that Bashir used "deceitful methods" to secure the interview by commissioning fake bank statements.
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Stevens regretted that he and his officers did not interview Bashir.
"If there'd been an allegation then that Bashir had produced allegedly fake documents to Princess Diana, which is a criminal offense, we'd have investigated it. My goodness me, we would have done," he said. "But this has only come out recently, which is unfortunate."