Brett Morgen reveals he found the rocker's last letter and, together with the research he did for the film, he's now sure Cobain shot himself because his personal and professional lives were unravelling before him.

In a new NME interview, the filmmaker says, "I found his actual suicide letter, which was unexpected. I just opened up this heart shaped box and there it was. And I was like, 'Based on everything I've witnessed, this makes no sense'. Kurt didn't have a problem with quitting music; he talked about it openly. If he wanted to stop performing, he would just stop. Nothing was leading up to that suicide letter.

"It was ultimately about his search for family. He didn't feel like he had a family, but he desperately wanted one. So the second that Nirvana broke he asked Courtney (Love) to have a child. You're 25 years old, you're the biggest rock star in the world - having a baby isn't a very common reaction, you know?"

And it was one interview with his widow that appears towards the end of the movie that really gave Morgen a sense of Cobain's state of mind in the days leading up to his death, almost exactly 21 years ago (05Apr94). In the clip, Love reveals Cobain tried to kill himself in Italy a month before his death because he knew she was "thinking about" cheating on him.

The director adds, "You can see in the film he put all his eggs in one basket. And so when that (family) was broken in his mind, there was this sense of humiliation. Kurt Cobain died of a broken heart.

"He thought his wife cheated on him, and her and (daughter) Frances were all he had. The film invests a lot in trying to understand why Kurt would react the way he did to betrayal. We try to find the square root of it."

The documentary premiered at the Sundance Film Festival in January (15). It will hit U.S. TV screens next month (May15) before its global release.