In a new book, "Sinatra And Me: In The Wee Small Hours", Tony Oppedisano claims the singer believed Monroe was "silenced" for her links to President John F. Kennedy and his brother Robert.
"Frank believed she was murdered, and he never got over it," Oppedisano says, claiming Monroe was hanging out with her ex-husband, Joe DiMaggio, at the Cal Neva Lodge, which Sinatra co-owned, just before her death.
Sources tell The Independent Monroe was set to host a press conference at the hotel, which, according to the book, would revolve around a reunion with the baseball legend.
Monroe died before the conference took place, and Oppedisano claims Sinatra thought she was killed over fears she would use the event to share secrets about her affairs with the Kennedy brothers.
"Frank believed if the press conference hadn't been announced, she would have lived a lot longer," he says.
Monroe died in 1962 from an official drug overdose.
Elsewhere in the book, Oppedisano explains why Sinatra and Monroe were never been lovers. "Frank felt she was too troubled, too fragile, for him to sleep with and then walk away," he spills.
About the book itself, the author claims, "I tried to paint the portrait of a man very few people got to know as well as I did." He adds, "I think he knew someday I'd share the stories he wanted the world to know."