Frank Sinatra, the legendary actor and singer, reportedly never believed that the iconic star Marilyn Monroe actually died of an accidental overdose when she passed away at the age of 36 back in 1962. Instead, he was convicted that she was murdered, according to his former manager Tony Oppedisano.

“Frank believed she was murdered,” Oppedisano wrote in an excerpt of his upcoming memoir “Sinatra and Me: In the Wee Small Hours” that was obtained by People Magazine.

Monroe was found dead in her Los Angeles home on August 4, 1962, with her nude body being found faced down on her bed with a telephone in one hand, according to History.com. Though the Los Angeles Police Department ruled that the actress’ death was “caused by a self-administered overdose of sedative drugs and that the mode of death is probably suicide,” questions about her passing have remained all these years later.

Oppedisano alleged that Sinatra and Monroe “were close friends but not lovers.”

“Frank felt she was too troubled, too fragile, for him to sleep with and then walk away,” he explained.

Oppedisano went on to claim that Monroe confided in Sinatra about her affairs with President John F. Kennedy and his brother Robert F. Kennedy, which each 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 she died, Monroe was pictured at the Cal-Neva Lodge, which was partially owned by Sinatra and located just outside of Lake Tahoe. Oppedisano wrote that Monroe was in the area to spend time with her ex-husband, baseball star Joe DiMaggio, who was staying nearby. He added that Monroe had been telling people that weekend that she was preparing to make a press announcement the following week to declare that they were officially getting back together.

According to his former manager, Sinatra thought that the upcoming press conference led to 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 think she was actually going to go public with the relationships.

Sinatra’s attorney Mickey Rudin, who worked with Monroe, also reportedly thought she’d been killed. Rumors soon spread among mob boss Sam Giancana’s men, and some of them claimed involvement, according to Oppedisano. Sinatra reportedly heard from “several sources” that Monroe was indeed murdered.

“Conspiracy theories abound and I can’t lay them to rest,” Oppedisano admitted about her death, though he added that her passing “haunted” Sinatra for years.