LOS ANGELES , California ( CNN ) -- Someone stole an `` incomplete and early version '' of the next installment in the blockbuster `` X-Men '' movie series and posted it on the Internet this week , according to the studio that owns the billion-dollar film franchise . Hugh Jackman stars in `` X-Men Origins : Wolverine . '' An early version of the film was leaked to the Web . Twentieth Century Fox said the FBI was investigating who leaked `` X-Men Origins : Wolverine , '' which is set for release in U.S. theaters May 1 . The digital file quickly spread across the Internet and was available for free , but illegal , downloading from hundreds of easily found Web sites . `` The source of the initial leak and any subsequent postings will be prosecuted to the fullest extent of the law ; the courts have handed down significant criminal sentences for such acts in the past , '' the studio 's statement said . Watch how the leak might affect ticket sales » . Removing the pirated files from the Internet may prove an impossible challenge , an industry source said . This source did not want to be named because of the sensitive nature of the matter . Although the studio said it was removed from the Web site that first hosted it , the digital file has probably been downloaded tens of thousands of times , the source said . `` It 's not removed from the Internet , '' the industry source said . `` It 's still there . '' Twentieth Century Fox downplayed the value of the pirated copy , noting that it was not the final version . `` It was without many effects , had missing and unedited scenes and temporary sound and music , '' the studio said . Online movie reviewer Kent Lundblad said he was given a copy by a friend just hours after the rough `` Wolverine '' first appeared on a Web site Tuesday afternoon . `` Even in this rough form , it 's much better than the last ' X-Men ' outing or anything Fox has given us in a long , long time , '' Lundblad said in a review he wrote after watching it . He said the movie is a rough cut , using placeholder images in many sections and special effects that look like `` video games 12 years ago . '' He said he suspects that the final movie will look much different than the pirated version , with scenes that were re-shot . Lundblad , a 25-year-old who describes himself as `` a total nerd , '' said he will see the movie at its midnight premiere May 1 . He bought his ticket weeks ago online , he said . Steve Zeitchik , who covers the film industry for The Hollywood Reporter , said the studio 's biggest concern may be that having people view a rough cut `` completely throws the studios off their game '' of building a positive buzz for the movie . `` If people see this movie , and they do n't like it , and they tell their friends , and their friends blog about it , and it just spreads throughout the blogosphere , there are a lot of people that do n't even get near a pirated copy of this film , who do n't go see [ the movie in theaters ] because of this leak , '' Zeitchik said . Whether the leaked video will eventually hurt the film 's box office earnings `` is very difficult to discern , '' according to CNN 's source . The latest `` X-Men '' film stars Hugh Jackman as Wolverine . Investigators have several tools to help find the source of the leak , including digital watermarking , the studio said . `` We forensically mark our content so we can identify sources that make it available or download it , '' the studio said . An FBI spokeswoman confirmed that the agency is `` working with the studio and investigating the alleged online compromise of the movie . '' The Motion Picture Association of America , the film industry 's main trade association , confirmed it was also helping in the probe .
NEW : Online reviewer saw early `` X-Men Origins : Wolverine , '' praises film . Twentieth Century Fox involved in investigation to see how `` Wolverine '' leaked . Latest film in Wolverine series had early version pirated , put on Web . `` `` Wolverine '' , '' starring Web , opens weeks ago .