Sir John Stevens, an investigator into the car accident that killed Princess Diana nearly 25 years ago, recalls the emotional discussions he had with Princes William and Harry over the tragic events of his mother's death on August 31, 1997, at the age of 36.

"Yes, of course it was difficult," Stevens told Us Weekly ahead of the release of the Discovery+ documentary "Princess Diana Investigations. "I was outside the door [of Kensington Palace] and they said, 'No, they only want to see you.'"

Stevens says he spoke with the boys for over an hour.

"I don't want to divulge any further conversation about the circumstances of their mother's death, when and where she died, what she said," he says.

Stevens is a former police chief who led the years-long investigation into the death of the late Princess of Wales. He called the discussion with William and Harry "very emotional" and noted that he, too, was emotional about the accident. He told the magazine that his sons were understandably very upset about the circumstances of the princess's death in Paris.

"What they were angry about, and they professed it, were the paparazzi, who they strongly believed were behind what happened by chasing the car and getting it to go 75 miles per hour, or leading it to go 75 miles per hour," Stevens said. Stevens says, "It was the paparazzi who thought they were behind what happened by chasing the car and getting it to go 75 miles per hour, or by directing it."

"The Diana Investigation Unit" will premiere on Discovery+ on August 18 and will provide viewers with new insights into Diana's death through new interviews with the lead investigators on the case, including Stevens.

"The bottom line is that we worked on this case for three years, and every single aspect of what we did, every single allegation we made down to 104 allegations, and we investigated every single one of them," Stevens said. 'That's one of the reasons we did this program. We're doing the program with people we trust.