For decades, Fred Otash was alleged to have kept the darkest secrets of Hollywood stars, including America’s most famous sex symbol.

Tinseltown’s most notorious private detective died in 1992 at age 70. The World War II Marine veteran is the subject of a new book, "The Fixer: Moguls, Mobsters, Movie Stars, and Marilyn." 

It delves into shocking revelations from his never-before-seen investigative files.

For the book, co-author Manfred Westphal was given access to Otash’s archives with the blessing of his daughter Colleen. Westphal, who first met Colleen at Otash’s funeral, developed a close friendship with her over the years.

The Fixer book cover

Josh Young and Manfred Westphal have written a new book about late private investigator Fred Otash, "The Fixer." (Grand Central Publishing)

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"One of the driving forces was to tell the story of his life, the cases he was involved in and what really happened — what actually happened," co-author Josh Young told Fox News Digital. 

Fred Otash demonstrating his wiretapping equipment

Fred Otash died in 1992. He was 70. He is seen here holding a wristwatch microphone at his desk at Fred Otash Detective Bureau, circa 1960. (© the Otash/Houck Archives)

"Not only was this a chance to do that, but also a chance to delve into this fascinating world of this extraordinary … larger-than-life character who walked out of a Raymond Chandler novel," Young shared. "He was involved in the golden heyday period of Hollywood in the ‘50s and ‘60s with all the biggest stars, mobsters and studio executives."

"[Colleen] granted us unfettered access to his never-before-seen personal archives, which served as the primary source for the book," Westphal told Fox News Digital. 

"In essence, he’s speaking from the grave."

Split side by side image of Marilyn Monroe and JFK

Fred Otash previously alleged he had audio recordings that proved Marilyn Monroe had an affair with President John F. Kennedy. (Murray Garrett/Bettmann/Getty Images)

According to the authors, Otash was the son of Lebanese rug merchants whose life was marked by tragedy. 

He lost his father and only brother during the Great Depression, prompting him to drop out of high school at age 16 to join the Civil Conservation Corps to support his mother and sisters. He went on to volunteer for the Marine Corps and, at the outset of WWII, fought in the South Pacific. 

Fred Otash posing in his LAPD uniform

Fred Otash’s LAPD head shot, circa 1947. (© the Otash/Houck Archives)

Otash landed in Los Angeles in 1945, where he signed up for the LAPD. There, he made a name for himself as a renegade cop before launching his own detective bureau in 1955 that, thanks to his access to emerging technology, eclipsed his contemporaries. He served as a freelancer for the LA tabloid Confidential.

A side profile photo of John F. Kennedy and Jackie Kennedy

President John F. Kennedy with wife Jackie Kennedy, who reportedly was not present when he first met Marilyn Monroe. (Getty Images)

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"Fred equalized the playing field for a lot of people," Young explained. "The LAPD and the FBI were wiretapping celebrities and getting information that way. But Fred had his own wiretapping system that was just as sophisticated as theirs, if not more. … He busted a Hollywood casting couch early on in his career where a producer was taking advantage of a young [girl] who came to town.

A close-up of Marilyn Monroe wearing a gold halter dress

Marilyn Monroe is still recognized as Hollywood's most famous sex symbol. (Frank Povolny/Twentieth Century Fox/Sunset Boulevard/Corbis via Getty Images)

"He came to the aid of many celebrities during that time when they had nowhere else to go and conventional law enforcement was not an option."

In 1985, Otash alleged to the Los Angeles Times he was hired by Peter Lawford to "do anything to remove anything incriminating" about the actor’s brothers-in-law, President John F. Kennedy and Senator Robert F. Kennedy. Both Kennedys had been rumored to have romantic liaisons with Monroe. 

A glamorous photo of Jayne Mansfield

Fred Otash alleged that President John F. Kennedy also had a brief fling with Jayne Masnfield, a Hollywood blonde bombshell and Marilyn Monroe's rival. (20th Century-Fox/Getty Images)

In the book, Otash alleged that he heard the president having "sexual congress" with the blonde bombshell.

"Yes, these were audio recordings," Young alleged. "Those were made at Peter Lawford’s Santa Monica beach house. And they were made through a series of circumstances that I’ll let the readers delve into in the book."

The book alleged that Monroe’s affair with Kennedy was a secret that Lawford was determined to keep from the hungry press. Otash described Lawford as "Jack Kennedy’s sexual archivist." 

Lawford’s beach house, the book alleged, was known as Kennedy’s crash pad for having affairs with numerous women, including sought-after actresses.

Marilyn Monroe singing happy birthday at Madison Square Garden

Actress Marilyn Monroe singing "Happy Birthday" to President John F. Kennedy at Madison Square Garden for his 45th birthday. (Getty Images)

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"Marilyn Monroe was certainly the most famous movie star ever recorded having sex with Kennedy, but she wasn’t the first," the book alleged. "That dubious honor went to fellow bottle blonde actress Jayne Mansfield. A close friend of Lawford, ‘The Girl Can’t Help It’ star was considered by some to be Monroe’s competition at 20th Century Fox."

Fred Otash showcasing his wiretapping equipment

Fred Otash poses with an array of his surveillance equipment at Fred Otash Detective Bureau, circa 1956. (© the Otash/Houck Archives)

In his own recordings, Otash noted, as quoted by the book, "Monroe became the central figure. The original electronics surveillance had nothing to do with her. But now, it was established that Monroe was f------ Jack Kennedy. She became the one, the main figure, because she was the best known."

Young claimed the audio recordings proving that Kennedy and Monroe had an affair went missing.

Marilyn Monroe and President John F. Kennedy

The only known photo of Marilyn Monroe and John F. Kennedy together was taken during his birthday celebration at Madison Square Garden. (ALAMY)

"We believe that they were destroyed," he explained. "No one knows for certain but … they were turned over to one of Fred’s confidants in the LAPD. We detail in the book what he thinks might have happened to them, but he was never actually certain where they ended up. He wondered if they possibly ended up in the attorney general’s office. But, again, that was something he didn’t know."

According to the book, Monroe first met Kennedy in 1955 at a party at the home of Charles Feldman, producer of her blockbuster film "The Seven Year Itch." Joe DiMaggio, her husband at the time, had no desire to attend the bash, nor did he want his wife there. 

Marilyn Monroe trying to hold down her flowing white dress

Actress Marilyn Monroe tries to hold down her dress as wind from a subway grate blows it upward during the filming of "The Seven Year Itch" in New York City. (Getty Images)

While Kennedy was in town to visit his sister and brother-in-law, his wife Jackie Kennedy stayed behind in Boston. The book described how Kennedy would allegedly "go hunting," or "bed Hollywood starlets" while his wife was away. Kennedy’s meeting with Monroe that evening was platonic, and it was allegedly two years later that they had an affair at Lawford’s home.

A close-up photo of Peter Lawford smiling in a grey suit

Peter Lawford (1923-1984), a British actor, was married to Patricia Kennedy from 1954 to 1966. (Silver Screen Collection/Getty Images)

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Otash, who kept private surveillance on Monroe, also alleged that he heard the blonde bombshell die. He claimed that Monroe called Lawford twice, then Kennedy once, from her home in August 1962. She then called Lawford once more and purportedly said, "Say goodbye to [your wife] Pat. Say goodbye to the president and say goodbye to yourself because you’re a nice guy." 

Marilyn Monroe walking next to a man in a suit

"Say goodbye to Pat. Say goodbye to the president and say goodbye to yourself because you’re a nice guy," Marilyn Monroe allegedly told Peter Lawford in her final phone call to him, as claimed by Fred Otash. (ALAMY)

After the call, Lawford rushed over to Monroe, where he found her dead. Otash alleged that instead of calling the police, Lawford "started rummaging around, trying to pick all the s--- he could that would implicate anyone." Otash later told friends he "always knew Lawford was weak, but to do nothing to help a so-called dear friend that might still be alive was beyond imagination."

John F. Kennedy talking to his sister Patricia while Peter Lawford listens

John F. Kennedy speaks with his sister Pat as Peter Lawford listens. (Getty Images)

"Fred had Marilyn’s house on Fifth Helena Drive tapped for a variety of reasons that we go into in the book," Young explained. "And on the night that she died, the wires were active. We reveal what Fred heard on those audio tapes, which goes into the mystery of what happened the night that Marilyn died.

"I think, for me, it adds another piece to the puzzle," Young admitted. "Marilyn’s death is one of those things that has been mythologized, misconstrued and written about … even many of the people who were involved at the time. And those who claimed they went to her house after she died later changed their stories for profit. 

Fred Otash holding his baby daughter

Fred Otash with his six-month-old daughter Colleen, December 1949. (© the Otash/Houck Archives)

"I think it was nice to have a little bit of a slice of what may have gone on. Fred himself made recordings of his experiences. It was through those tapes that we were able to glean his quotes of what he said happened that night."

A close-up of Marilyn Monroe smiling in a white and floral dress

Marilyn Monroe during her last movie "Something's Got to Give" directed by George Cukor, which remained unfinished.  (AFP via Getty Images)

Young believed Otash may have had regrets about Monroe’s demise.

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Marilyn Monroe looking somber

Marilyn Monroe, seen here in 1955, died in 1962. She was 36. (Hulton Archive/Getty Images)

"Ultimately, he saw how she was swallowed up by the system," Young explained. "He saw what it did to her. … I don’t know if ‘regret’ is the exact word. … But what happened to her affected him. … And those feelings evolved not only over the time that he knew Marilyn but also after her death."

Monroe died in 1962 from a barbiturate overdose in her home. She was 36.