Lisa Marie Presley's son Benjamin Keough had cocaine and alcohol in his system at time of his suicide and had attempted to take his own life before.
Benjamin died in July aged 27 leaving his family devastated.
He was found unresponsive in the bathroom of his mother's home in California during a party celebrating his girlfriend Diana Pinto's birthday.
Emergency services were called and he was pronounced dead at the scene.
Diana is believed to have picked the lock on the bathroom door and found her boyfriend on the floor.
An autopsy report, obtained by The Sun, shows he had taken cocaine and consumed alcohol prior to his death.
It also revealed he "had a history of depression" and had made a previous attempt to take his own life between five and six months earlier.
Benjamin - the grandson of music legend Elvis Presley - had been admitted to rehab three times but is said to have only completed one of the stints.
After his death, the Los Angeles County Coroner's Office ruled his death a suicide and revealed he died from a self-inflicted gunshot wound.
Benjamin was buried alongside his grandfather Elvis Presley at Graceland, the King's estate in Memphis, Tennessee.
The news was confirmed in a post on the official Graceland Facebook page.
The statement read: "Benjamin Storm Presley Keough was laid to rest in the Meditation Garden at Graceland with his family including his grandfather, Elvis Presley, great-grandmother Gladys Presley, great-grandfather, Vernon Presley and great-great-grandmother, Minnie Mae Presley."
Lisa Marie was the King's only child, and she became mum to Benjamin and Riley with her ex-husband Danny Keough.
She also has twins, Harper and Finley, with fourth husband Michael Lockwood, who she split from in 2016.
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