Journalists Jay Margolis and Richard Buskin in their book The Murder of Marilyn Monroe: Case Closed, repeated the story about Marilyn and Robert F. Kennedy having an affair and then getting into an argument the day before her body was found.
According to this theory Kennedy told Marilyn that he wouldn’t marry her, and Marilyn threatened to stage a public conference and reveal her affairs with him and his brother John. Kennedy demanded that Marilyn leave him alone, and to hand over the diary in which she kept track of her affairs. Marilyn refused, and Kennedy left in a fit of rage.