Three years ago, a biographical documentary was released to commemorate the 20 year anniversary of Princess Diana’s Aug. 31, 1997 death. The contents of the film reveal Prince William and Prince Harry‘s deep-seated regrets over their last conversation with their mother.

On Monday, the 23-year anniversary of the tragic death of the beloved Princess Diana, an ITV documentary recalled a very revealing picture about her two surviving sons’ remorse over what was left unsaid. Diana, Our Mother: Her Life and Legacy was released on August 31, 2017, to coincide with that year’s anniversary, and it contains perhaps the most intimate interviews with the two princes. The members of the royal family even went so far as to tell People magazine that they would never be so candid regarding their mother again.

In the film, the two brothers express their remorse over not lingering a little longer on their last phone call with her. As they were just young boys then and eager to play with their cousins, they did not consider that they would never get a chance to speak to their mother again. “If I’d known that that was the last time I was going to speak to my mother the things that I would – the things I would have said to her,” Harry said.

Prince William said of the biopic, “We feel hopefully this film will provide the other side from her close family friends that you might not have heard from before, from those who knew her best, and from those who want to protect her memory and want to remind people of the person that she was: The warmth, the humor and what she was like as a mother, which probably hasn’t come across before in many other pieces from other people.”