Private detective believes portions of the note may have been written later

'Practice sheet' contains key letter combinations also found on the note

He was found dead in his home two decades ago after an apparent suicide.

But over twenty years on from Nirvana frontman Kurt Cobain's shocking death, a new documentary attempts to uncover fresh clues, going as far as to suggest the iconic musician was murdered.

In MailOnline's exclusive clip from Benjamin Statler's anticipated film Soaked In Bleach, experts are seen dissecting Kurt's suicide note, which throws up a dramatic set of questions in the investigation.

Scroll down for video

New questions: Benjamin Statler's anticipated film Soaked In Bleach examines the death of Nirvana frontman Kurt Cobain, who according to the official version of events killed himself in April 1994

Former Seattle police chief Norm Stamper, who stood at the helm of the Seattle Police Department from 1994 to 2000, is front and centre of Statler's film as he declares the investigation into Kurt's death should be reopened.

According to the official version of events, the Nirvana singer killed himself with a shotgun blast to the head on April 5, 1994, inside his home near Lake Washington, in the Seattle area.

But the volatile musician's untimely, violent demise has been the subject of intense speculation -giving rise to the idea that Kurt was murdered.

Theory: Norm Stamper (left), who was chief of the Seattle Police Department from 1994 to 2000, says officers should take another look at all the evidence before reaching a final conclusion about Cobain, pictured in 1993

Blame: Courtney Love and Kurt Cobain with their daughter Frances, who was born in 1992. Courtney has been trying to stop the general release of the film, which claims she was somehow involved in Kurt's death

In the new documentary, Stetler presents the case from the point of view of private investigator Tom Grant, who was hired by Cobain's wife Courtney Love to find the missing musician just days before he turned up dead.

The new teaser points the blame at Courtney, as Heidi Harralson, a forensic document examiner presents her findings from a detailed look at the suicide note which Courtney has kept under lock and key in a safe deposit box since her husband's death.

Heidi explains that the investigation discovered a 'handwriting practice sheet' found in Courtney's bag which contained different letters of the alphabet, and was compared to the suicide note.

Investigation: In MailOnline's exclusive clip experts present their findings from a detailed look at Kurt's suicide note which Courtney has kept under lock and key in a safe deposit box since her husband's death

The practice sheet found in Courtney's belongings contains 'letter combinations of specific letters that are found in the bottom portion of the suicide note,' explains Heidi in the clip.

'It is possible that someone else with some skill could indeed imitate his writing, especially those last few lines,' she claims.

Carole Chaski, a forensic linguist, adds to the theory by pointing out that the top portion of the long note, as well as appearing to be in a different handwriting, is also of a different style linguistically to the bottom four lines.

Fresh evidence: Heidi Harralson, a forensic document examiner explains that a handwriting practice sheet was allegedly found in Courtney's possession

Match: The practice sheet found in Courtney's belongings contains 'letter combinations of specific letters that are found in the bottom portion of the suicide note,' explains Heidi in the clip

The main body of the note, which is addressed to Cobain's childhood imaginary friend "Boddah", is written about Kurt's 'relationship to music, it's only the very small part of the note, the last four lines, that talk about his relationship to his family,' and specifically Courtney and their daughter Frances Bean, explains Carole.

'That is what most of us would consider a stereotypical suicide note "I love you, someone will be better off without me. Keep moving forward."'

'Those are the kind of things we expect to find in suicide notes or even in overkill, to make it very stereotypical so that people couldn't miss what it is.'

The note: The main body of the letter, which is addressed to Cobain's childhood imaginary friend "Boddah", is written about Kurt's 'relationship to music

Differences: Carole Chaski, a forensic linguist, adds to the theory by pointing out that the top portion of the long note is also of a different style linguistically to the bottom four lines

Change: 'It's only the very small part of the note, the last four lines, that talk about his relationship to his family,' and specifically Courtney and their daughter Frances Bean, explains Carole.

Tom Grant, the private detective who was interviewed for Soaked In Bleach, has argued that the letter reads as a note announcing Cobain is leaving his wife and retiring from the music business - and not as a farewell to life.

He has said that the final few lines at the bottom - which do imply suicide - were added afterwards by an unknown individual.

Stamper is even more explicit in his view that those close to Kurt should be questioned further. ‘We should in fact have taken steps to study patterns involved in the behavior of key individuals who had a motive to see Kurt Cobain dead,’ Stamper says in the documentary, according to Spin Magazine.

Shock death: According to the official findings the musician killed himself with a shotgun blast to the head on April 5, 1994, inside his home near Lake Washington, in the Seattle area

Plea to reopen: Stamper argues that those close to Kurt should be questioned further

‘If in fact Kurt Cobain was murdered, as opposed to having committed suicide - and it was possible to learn that - shame on us for not doing that. That was in fact our responsibility.’

Stamper says he is not convinced that the 27-year-old was the victim of foul play, but he would like police to take another look at all the facts before reaching a final conclusion.

'It's about right and wrong. It's about honor. It's about ethics,' he argues. 'If we didn't get it right the first time, we damn well better get it right the second time.'

The 1998 documentary Kurt & Courtney, by filmmaker Nick Broomfield, also made allusions to Cobain's widow.

Happy family: No evidence has ever been found that Courtney, pictured with Kurt and their daughter Frances in 1993, was in any way responsible for her husband’s death

No evidence has ever been found that the now 50-year-old female rocker was in any way responsible for her husband’s death.

And in fact, Courtney has been trying to stop Soaked in Bleach from being released, Deadline have reported.

In June, her legal team issued a cease-and-desist order to prevent movie theaters from showing the controversial documentary, which was not authorized by her late husband's estate.

‘The Film falsely presents a widely and repeatedly debunked conspiracy theory that accuses Ms. Cobain of orchestrating the death of her husband Kurt Cobain,’ the singer’s attorney wrote in the order.

Soaked in Blood is released on DVD and digital on 19th October.