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The protectors of Marilyn Monroe's image say a collector who wants $500,000 for a short 1940s stag film is committing fraud and violating the actress's intellectual property rights by claiming it shows her having sex when she was underage.

Mikel Barsa, a Spanish events promoter, insists that the scratchy, black-and-white six-minute film shows the young actress, known then as Norma Jeane Baker, around 1946 or 1947 when she was poor and desperate to break into show business.

Experts on Monroe's life, however, say it's highly unlikely that the smiling young blonde in the film is her. Comparing the film with known Monroe images leaves ample room for doubt. Also, while Barsa shows off various documents he says support his theory, they self-evidently fall short of proof.

Whoever the woman in the film may have been, even alleging that it shows Monroe violates her intellectual property rights and will cost Barsa dearly if he goes ahead with the sale, said Nancy Carlson, a spokeswoman for the brand development and licensing company Authentic Brands Group.

Barsa is inviting legal action for "perpetrating a fraud on the public, violating the Monroe estate's exclusive rights to her image and other claims of intellectual property infringement," Carlson said in an interview.

Barsa said he'll broker the sale on Sunday evening anyway and collect his 10 percent from the film's owners, whom he refused to identify.

"It always is the same story when it comes to Marilyn — to deny, deny, deny and to threaten," Barsa said.

Barsa claims the 8 mm strip of celluloid was made simultaneously with a 16 mm version, which he claims he sold for $1.2 million in 1997. He refuses to identify any of the sellers or buyers of the films, but said a European magazine also bought rights to make copies and later sold 600,000 videotapes promoted as documenting Monroe's secret sex life. Copies of the tape can still be found online.

Barsa offers a 1996 letter from Alan Brown of the nonprofit American Film Institute that he say confirms the woman is Monroe. But the letter says only that "it's not clear whether the woman in the film is Marilyn Monroe — if not, she's definitely a lookalike." A spokesman for the American Film Institute declined to comment again on the controversy.

Barsa also shared a copy of what he said was a declassified FBI document proving Monroe was in the film. The heavily redacted text, however, refers to thirdhand information from a source who claimed Monroe's ex-husband, baseball great Joe DiMaggio, offered $25,000 in 1965 for a film that someone else claimed showed Monroe having sex. The document itself doesn't say if the alleged film existed.

Barsa said he guesses the film was made in the months before Monroe signed on with movie studio 20th Century Fox in 1947.

"Marilyn had no motivation at all to make a porn movie," countered Michelle Morgan, who wrote "Marilyn Monroe: Private and Undisclosed."

"Her friends all agree that she never, ever showed any signs of having earned money in any other way than modeling or working as a real actress," Morgan said.

Monroe "wasn't a prude," said Lois Banner, who wrote "MM-Personal: From the Private Archive of Marilyn Monroe." But Banner also considers it extremely doubtful that financial difficulties would have put her in such a spot. Even at her poorest, in 1947, Monroe had a wealthy couple helping out by providing her with $100 a week, she said.

"She had a very purple sex life at times. But she never made any money being a party girl. She did it to advance her career," Banner said.

And Scott Fortner, a Monroe fan who maintains a blog about her and collects the star's clothing, has sent the AP a series of pictures pointing out differences between the star and the woman in the film, from her teeth to her chin to her ears. Some show that while Monroe had a prominent widow's peak, the hairline of the woman in the sex tape seems straight.

Monroe underwent plastic surgery in 1950 to change her nose and chin, but her widow's peak never changed, Fortner said.

"To me personally, it doesn't even resemble her," Carlson said.