Previously unpublished documents have revealed the scale of a plot by the media mogul William Randolph Hearst to discredit Orson Welles and destroy Citizen Kane, the 1941 film about the rise and fall of the fictional newspaper proprietor Charles Foster Kane.

Welles and RKO Pictures faced extortion, media manipulation and other underhand tactics in a plot that was much darker and began earlier than was previously known, according to research by Harlan Lebo for a forthcoming book.

The book cites a memo of 1941, in which Welles’s lawyer-manager, Arnold Weissberger, warned his client that the Hearst empire would stop at nothing: “This is not a tempest in a teapot, it will not calm down, and the forces opposed to us are constantly at work.”