Kurt Cobain was found with a lethal gunshot wound to the head on April 8, 1994 at his Seattle, Washington home.

The Seattle Police Department (SPD) took over the investigation, finding a suicide note, a gun in his hands and a nearby heroin kit, which they used to determine the Nirvana frontman, 27, had taken his own life.

Now, in a newly released update tied to his controversial book Case Closed: The Cobain Murder: The Killing and Cover-Up of Kurt Cobain, author Ian Halperin, a journalist known for several bestselling investigative books, claims that an unnamed former Seattle police detective privately told him the investigation into Cobain's death was mishandled. 

Halperin wrote that he spoke to the former police officer about 18 months ago and quoted the source as saying: 'I have felt a sense of righteous indignation for years. Finally, I decided to speak out to address a serious wrongdoing.'

Halperin said the unnamed former detective claimed Cobain's heroin levels were so high that it would have been impossible for him to fire the gun himself. 

'There were no fingerprints on the gun, and the last five lines of his alleged suicide did not match his own handwriting,' the source said, according to the book.

'Just on that, the case should not have been labeled a suicide. A proper investigation should have been conducted, a thorough investigation to find out how, in fact, Cobain died. It was never done.'

When asked about a potential mishandling of Cobain's death, a SPD spokesperson told the Daily Mail: 'Kurt Cobain died by suicide in 1994. This continues to be the position of the Seattle Police Department.'

The former SPD office also claimed there was 'poor management,' with Halperin writing that the source said they were among several people within the department who believed the entire investigation was 'one big cover-up.'

Halperin wrote that the source worked under Norm Stamper, the Seattle Chief of Police from 1994 to 2000, who has expressed regret over how Cobain's case was handled, stating in 2015 that he 'would reopen this investigation' if given the opportunity.

While not directly accusing anyone, Stamper previously suggested that investigators should not have immediately discounted the possibility of murder.

Halperin's source echoed Stamper's concerns, saying: 'Too much politics was involved. Not enough facts. Many people were able to advance themselves at others' expense. As a result, many cases were treated unfairly.' 

Halperin wrote that when asked why the unnamed former detective questioned the suicide ruling, the source cited several concerns about the investigation. 

Similar concerns have been raised in previous years by other law enforcement figures who later reviewed the case.

In a previous interview with the Daily Mail, retired Seattle Police Captain Neil Low, who was asked by his chief to audit the Cobain case in 2005, said he believed investigators failed to properly treat the death as a potential homicide.

'I just am not buying that Kurt did that to himself,' he said, describing the investigation as 'botched.' 

However, Low was not involved in the original investigation and reviewed the case years later. 

The SPD has repeatedly maintained that Cobain died by suicide in 1994, a conclusion that remains its official position.

Cobain died on April 5 from a shotgun wound to the head. His body was found three days later by an electrician installing security lighting at the house.

At the time, the King County Medical Examiner ruled his death a suicide by a Remington Model 11 20-gauge shotgun, as the weapon was found in his arms and a suicide note in a nearby potted plant.

According to Halperin, his source said: 'I was a detective for many years, and in this case it's quite easy to put two and two together.

'The killers incapacitated Kurt by forcing him to take a large dose of heroin. Then, they shot him in the head. To me, there's no other explanation. I studied this case thoroughly.'

The unnamed officer reportedly highlighted Stamper's statements, telling Halperin: '[Stamper] called for the case to be reopened, saying it wasn't properly investigated. 

'Stamper has 34 years of experience in law enforcement. He wouldn't say something like that unless he had hard facts and proof.'

This is Halperin's second book on the death of the legendary rock star, with 'Love & Death first published in 2004.

Halperin made a name for himself in 2009 when he published 'Unmasked: The Final Years of Michael Jackson,' which correctly predicted the singer's health decline and death. 

He told the Daily Mail that the copycat suicides are what motivated him to investigate Cobain's case.

'I've been in discussion with some of the families of the copycat suicide victims, and they deserve justice. If Kurt was murdered and didn't commit suicide,' he said.

The author began investigating the case, finding evidence collected at the scene did not point directly to an open-and-shut suicide case.

He noted that Cobain's toxicology showed he had injected roughly three times what would be considered a lethal dose for a typical user.

'Even for the most severe heroin addict, the amount found would have been lethal, estimated to be roughly 70 times the dose for an average person,' said Halperin.

'No human being could withstand that, and they would likely die within seconds. That's according to experts, not my own opinion.' 

He also said he hired handwriting experts to analyze the suicide note found at the scene, saying: 'They said the last five lines were not written in the hand of Kurt Cobain, which is the only thing that really dealt with suicide.'

'You know, again, and there were no latent fingerprints found on the gun. Dead men don't wipe their own fingerprints,' he added.

However, it was Stamper's admission that Halperin cites as 'proof' that the case should be reopened. 

Stamper was featured in the controversial documentary 'Soaked in Bleach,' an unauthorized film that scrutinizes Cobain's death. 

'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 said in the 2015 documentary.