In a case of piracy that some analysts called unprecedented, untold thousands of people watched a version of “X-Men Origins: Wolverine” online Wednesday, a full month before its scheduled theater release.
The film’s distributor, 20th Century Fox, said it did not know how the unfinished copy of the comic book adaptation was leaked onto the Internet. The copy was missing many special effects and included temporary sound and music.
Nonetheless, it circulated widely online beginning late Tuesday, even prompting some viewers to publish reviews, favorable and unfavorable, of the hotly anticipated film.
“Wolverine” stars Hugh Jackman in the title role and is set to open on May 1.
The troubling leak — which some people initially dismissed as an April Fool’s Day prank — occurred at a time when media companies are working harder than ever to curtail digital piracy of content.
Illicit recordings of films usually appear on the Internet shortly after their theater debuts, but leaks before the premiere dates are rare.
Hollywood studios spend millions of dollars to track every step of the film production process to avoid such potentially costly leaks.
Eric Garland, the chief executive of the file-sharing monitoring firm BigChampagne, called the widespread downloading of “Wolverine” a “one-of-a-kind case.” “We’ve never seen a high-profile film — a film of this budget, a tentpole movie with this box office potential — leak in any form this early,” he said.
The studio, a unit of the News Corporation , spent the day demanding that copies of the film be removed from the largely anonymous swath of Web sites that swap movie files.
But the copies propagated at such a swift rate that the digital cops could not keep up.
BigChampagne estimated the digital film copy had been downloaded in the low hundreds of thousands of times in its first 24 hours on the Internet.
The studio said the F.B.I. and the Motion Picture Association of America were both investigating the film’s premature distribution.
“The source of the initial leak and any subsequent postings will be prosecuted to the fullest extent of the law,” the company said, adding “the courts have handed down significant criminal sentences for such acts in the past.”
Media companies use watermarks and other technological strategies to identify the sources of leaks.
“Wolverine” is not the first film to receive an unintended preview on the Internet. Another superhero film, 2003’s “Hulk,” showed up as a download about two weeks before its release.
But the major studios hoped they had learned enough since then to keep it from happening again.
Mr. Garland said the existence of the illicit file could theoretically depress the box-office receipts for the film, but he emphasized that the online viewers would be only a tiny percentage of the total audience.
The “other fear is bad word of mouth,” he said.
As twisted as it may seem, “you would rather have a very high-quality version of the film leak than a premature working version of the film leak, because it’s not your best work.”
In the case of “Wolverine,” some of the computer-generated scenes were missing and other parts were unedited.
The studio noted that some fan Web sites condemned the leak. But other Internet users downloaded the file and weighed in with reviews.
“This is bad bad news for Fox,” a movie blog called In GOB We Trust said on Wednesday, asserting that negative comments about the film would reduce its box-office prospects.
But the blog reviewed the film anyway, saying that the creators decided to “dumb it down and essentially make a cartoon.”