After more than 50 years of speculation surrounding the sudden death of Hollywood icon Marilyn Monroe, a new documentary theorizes what killed the starlet was neither a suicide, nor a shady murder plot, but rather a negligent doctor.

Monroe, 36, was found dead in 1962 at her Los Angeles home with an empty bottle of sleeping pills near her bed. While a popular conspiracy theory ties her untimely death with her rumored romantic involvement with then-President John F. Kennedy and his brother, Attorney General Robert F. Kennedy, and other theories involve suicide or murder, a British documentary hopes to shed new light on the mystery.

Autopsy: The Last Hours of Marilyn Monroe enlists the U.K.’s leading forensic pathologist, Dr. Richard Shepherd to explore Monroe’s autopsy report, along with recently revealed private medical records, according to the Mirror.

While her death was officially blamed on a drug overdose, the documentary accuses her physician, Dr. Hyman Engelberg, who also performed her autopsy, of “signing her death sentence” by prescribing her a lethal cocktail of sleeping pills.

The Daily Mail reports Autopsy presents evidence showing that Engelberg lied under oath during a 1982 investigation about what drugs he had instructed Monroe to take prior to her death.

While Monroe famously suffered from insomnia and required a mix of sedatives and amphetamines to function, the documentary also claims she suffered from bipolar disorder and had a history of suicide attempts using powerful downers, making the doctor’s prescriptions of the powerful sedatives chloral hydrate and Nembutal reckless.

(Both chloral hydrate and Nembutal have been used during executions)

According to the Mail, the investigation concludes that Monroe had taken her normal dose of chloral hydrate before either accidentally or intentionally overdosing on Nembutal.

The documentary alleges that while her doctors were making an effort to reduce her drug dependency by slowly removing her off Nembutal and replacing it with the chloral hydrate, two days before her death, Engelberg changed course and renewed the prescription for Nembutal.

Dr. Shepherd said: “Chloral Hydrate and Nembutal taken together can have a fatal effect on the respiratory system. It’s curious that she should be given both drugs together.”

The noted forensic pathologist also stated: “She was a powerful, beautiful and seductive woman, used to getting her own way and I can imagine she would be very difficult to resist [if she was asking for the drug] but prescribing Nembutal on top of the Chloral Hydrate was like signing her death sentence.”

Shepherd concluded the information is the “final piece in the true story about the death of Marilyn Miss Monroe.”

No foul play is suspected in causing her death, but Autopsy claims the actress was known for having a volatile temperament, and a psychological evaluation from her autopsy report stated: “Miss Miss Monroe suffered from psychiatric disturbance for a long time, she experienced severe feats and frequent depressions. Mood changes were abrupt and unpredictable.”

The investigation claims that Monroe’s struggle with bipolar disorder took its toll on her career and personal life, and she attempted to commit suicide numerous times with sedatives, but always called for help before they took effect. She made an attempt to call for help on August 5, 1962 but passed out and died.

Dr. Hyman Engelberg died in Santa Monica, CA in 2005 at the age of 92.