Files that belonged to the late Fred Otash, a former Hollywood detective, have emerged and reveal details of the late Marilyn Monroe's death 51 years ago.

The documents, which were found by Otash's daughter Colleen in a storage unit, also contain information about the relationship the actress had with President John F. Kennedy and his younger brother Bobby.

According to the late Otash, who died in 1992, Monroe had a sexual relationship with the brothers.

His notes, which were published by The Hollywood Reporter magazine last week, contained details of what he heard via the bugging devices he installed in Monroe's Los Angeles home.

"I listened to Marilyn Monroe die," Otash wrote in his notes.

Shortly before his death, he told an interviewer that John F. Kennedy and Monroe "were having a sexual relationship ... but I don't want to get into the moans and groans."

Otash wrote that on August 5 1962, she had a violent argument with the Kennedys during which "she was...screaming and the two were trying to quiet her down."

"She's in the bedroom and Bobby gets the pillow and he muffles her on the bed to keep the neighbors from hearing. She finally quieted down and then he was looking to get out of there," he wrote of Monroe's last hours.

The private eye reportedly found out later that the famed Hollywood actress had died.

Otash then became the most famous detective in Hollywood and the first choice for any celebrity with a potential problem.

Crime novelist and author of LA Confidential James Ellroy met Otash a number of times. The writer used information from Otash's career in his novel Shakedown, which he published online. Ellroy is currently completing a script version for a TV show.

According to The Hollywood Reporter, the negative portrayal of her father in Ellroy's book led Colleen to search through her father's old files and make some of them public.

"I was very aggrieved," she said.

"[I thought]: what can we do to stop [Ellroy] from taking my father's life and turning it into just a horrible fictional depiction?" she added.

Otash died at the age of 70 in 1992.