The 'Kurt Cobain: Montage of Heck' documentary won't appear on HBO until May 4, but those in Park City, Utah got an early screening at the annual Sundance Film Festival. And surprisingly, there was one notable absence in the film -- Dave Grohl.

As expected, Kurt Cobain's parents, Courtney Love and fellow Nirvana founder Krist Novoselic all appear in the film, but Grohl was nowhere to be seen. However, director Brett Morgen says Grohl could be part of the version that will eventually air on HBO.

According to Morgen, Grohl was interviewed for the doc late in its process, so the filmmakers had a locked print with not enough time to edit Grohl in before the Sundance screening. "This is the picture I wanted -- I didn't want there to be a lot of people in the film," said Morgen at the 'Montage of Heck' premiere. "I hope we'll see a version [with the Grohl interview] some time."

Morgen told Billboard that Cobain's daughter, Frances Bean Cobain, was the key for the film. The Cobain offspring served as executive producer on the film, and Morgen adds, "She is the glue -- the movie's for her. Frances and I share the same exact vision of what this should be. People associated with Nirvana, Dave and Krist, wanted to do this for Frances."

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