Sony is investigating the possibility that a hacking attack on its film and TV studio may have involved North Korea, which has previously reacted with outrage to an upcoming Sony comedy about a CIA plot to kill leader Kim Jong-un, a source said. A person with knowledge of the matter said that the studio may have had several films stolen in the November 25 cyberattack that continues to cripple its computer systems. The recent picture Fury , a Brad Pitt war movie, and the yet-to-be-released Annie and Still Alice have appeared on file-sharing sites, said the person, who sought anonymity because of the sensitive nature of the matter. The website TorrentFreak, a news site on file-sharing, said Fury was the second most-downloaded film at one site. The source said the studio was looking into Pyongyang's possible involvement in the attack. The website Re/code reported on Saturday that the studio was looking into whether the attack was intended to coincide with the release of The Interview . The comedy, currently being advertised for release in theatres on December 25, features Seth Rogen and James Franco as trash TV producers who are recruited by the CIA to assassinate Kim after he invites them to interview him. Plans for the film drew a rebuke from the country, with a foreign ministry spokesman saying in state media that the release would be an "act of war", according to the BBC. In a statement last week, Sony said it was investigating "an IT matter". The group believed to be behind the attack is threatening further releases of data, the person confirmed. Sony Pictures called the theft of Fury , Annie and Still Alice a "criminal matter" in an e-mailed statement on Sunday, and said the company is working with law enforcement to address it. There was no response to two phone calls and a voice message seeking comment from Sony's Tokyo offices outside business hours. The hacking incident at Sony Pictures Entertainment began when a picture of a skull appeared on company computer screens. The image was accompanied by a message that read Sony had been hacked by GOP, believe to stand for "Guardians of Peace", and that private data would be released if undisclosed demands weren't met. The attack has forced some employees to communicate via text message and private e-mail accounts. Some systems at the entertainment division of Tokyo-based Sony have been brought back online, according to the person with knowledge of the matter. Email and some other systems remain down. The studio's home entertainment division was able to fulfil Black Friday orders, the person added. Vast libraries of films and TV shows are the main assets of studios such as Sony. Online theft, along with pirating of DVDs, costs the industry billions of dollars annually in lost ticket sales or retail purchases. The remake of the 1982 Annie is Sony's next big film release, schedule to hit theatres on December 19. The new version stars Quvenzhané Wallis, Cameron Diaz and Jamie Foxx. One comfort for the studio is that the targeted audience for Annie doesn't often use pirated content, the person said. The website BoxOffice.com predicts Annie will generate US$100 million for its run in US and Canadian cinemas. Films frequently do much more in overseas markets. Still Alice is considered a possible Oscar contender for lead actor Julianne Moore, according to the website GoldDerby.com . Last week's cyberattack isn't the first for Sony. In 2011, Sony's online-entertainment service was targeted by hackers who gained access to information on millions of customers.