Frank Sinatra didn’t believe that Marilyn Monroe died from an accidental overdose.

The claim is being made by his former manager Tony Oppedisano in his memoir "Sinatra and Me: In the Wee Small Hours," which is scheduled to be released on June 8.

"Frank believed she was murdered," Oppedisano wrote in an excerpt obtained by People magazine on Wednesday.  

The actress was found dead on August 4, 1962, at age 36. Her body was discovered nude on her bed, face down, with a telephone in one hand, History.com shared. According to the outlet, empty bottles of pills, prescribed to treat her depression, were littered around the room.

The Los Angeles police concluded that Monroe’s death was "caused by a self-administered overdose of sedative drugs and that the mode of death is probably suicide." However, the circumstances surrounding her passing still raise questions decades later.

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Marilyn Monroe was found dead on August 4, 1962, at age 36.

Marilyn Monroe was found dead on August 4, 1962, at age 36. (Getty)

According to Oppedisano, the movie star and singer "were close friends but not lovers."  

"Frank felt she was too troubled, too fragile, for him to sleep with and then walk away," Oppedisano wrote, as quoted by the outlet.

The author alleged that Monroe confided in Sinatra about her affairs with President John F. Kennedy and his brother Robert F. Kennedy, which ended abruptly.

"Marilyn told Frank she didn’t understand why they’d shut her out completely once she stopped having sex with them," he alleged.

The weekend before her death, Monroe was photographed at the Cal-Neva Lodge, outside of Lake Tahoe, which was partially owned by Sinatra.

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Frank Sinatra confidant Tony Oppedisano wrote a book titled ‘Sinatra and Me: In the Wee Small Hours.’

Frank Sinatra confidant Tony Oppedisano wrote a book titled ‘Sinatra and Me: In the Wee Small Hours.’ (Photo by Martin Mills/Getty Images)

Oppedisano claimed Monroe was in town to spend time with her ex-husband, Joe DiMaggio, who was staying nearby. He claimed that Monroe was gearing up to make a press announcement the following week to declare that they were officially getting back together.  

Oppedisano believed the news of a press conference sparked rumors that Monroe was going to expose her relationships with the Kennedy brothers, who were both married.

"Frank believed if the press conference hadn’t been announced, she would have lived a lot longer," he wrote, adding that Sinatra didn’t believe Monroe would have detailed her relationships with the Kennedys to the press.

He also noted that within days after Monroe’s death, Sinatra’s attorney Mickey Rudin, who also worked with her, believed she had been killed. He claimed that the rumor then circulated among mob boss Sam Giancana’s men, and some of them claimed involvement. The story went back to Sinatra, who heard it from "several sources."

For decades, numerous theories about Monroe’s death have persisted, ranging from the mafia being involved to the Kennedys. Some have also claimed the blonde bombshell’s death was a cover-up by her own doctor. However, none of these assumptions have ever been proven.  

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FILE - In this June 2, 1955 file photo, actress Marilyn Monroe, right, dressed in a glamorous evening gown, arrives with Joe DiMaggio at the theater. DiMaggio's love letter to Marilyn Monroe has sold for $78,125 at a Beverly Hills auction. Julien's Auctions in Beverly Hills says the letter, written by the baseball great after Monroe announced she was divorcing him, was sold Saturday, Dec. 6, 2014, to an undisclosed buyer.(AP Photo, File)

Joe DiMaggio and Marilyn Monroe were married from 1954 until 1955. (The Associated Press)

"Conspiracy theories abound and I can’t lay them to rest," Oppedisano admitted.

Still, Monroe’s death "haunted" Sinatra over the years, the author has claimed.

Shortly before her passing, Monroe and DiMaggio were spending time together. The baseball legend even told friends that they were going to get remarried, PBS.org reported. According to the outlet, it was DiMaggio who authorities called to notify him of her death. He then stepped in and orchestrated his ex-wife’s funeral.  

DiMaggio barred Hollywood producers, directors and stars like Sinatra from attending.  

"Tell them if it wasn’t for them, she’d still be here," said DiMaggio, as quoted by the outlet.

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Conspiracy theories about Marilyn Monroe's death continue to persist today.

Conspiracy theories about Marilyn Monroe's death continue to persist today. (Photo by Alfred Eisenstaedt/Pix Inc./The LIFE Picture Collection via Getty Images      )

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For two decades, DiMaggio had flowers delivered to Monroe’s grave twice a week, the outlet shared. He passed away in 1999 at age 84.

Sinatra passed away in 1998 at age 82.