SAN FRANCISCO (CBS/AP) — The FBI has confirmed it is investigating a recent hacking attack at Sony Pictures Entertainment, which reportedly caused major internal computer problems at the film studio last week.

Sony’s corporate email and other internal systems were knocked offline, according to reports by Variety and other trade publications. Sony workers reportedly saw a message appear on their computer screens that said “Hacked by #GOP,” which may be the initials of a group calling itself Guardians of Peace.

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Copies of some unreleased Sony films such as “Still Alice,” ”Annie,” ”Mr. Turner,” and “To Write Love on Her Arms” are now being distributed on unauthorized file-sharing websites, although a direct connection to the hacking hasn’t been confirmed.

Sony is reportedly investigating whether the hackers have ties to North Korea, whose leader Kim Jung Un is reportedly incensed by the assassination plot of Sony Pictures’ new comedy, “The Interview.”

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The FBI said in a statement that “the targeting of public and private sector computer networks remains a significant threat.”

Computer systems at the studio have been down since the attack last week, and employees say they have had to do things the old fashioned way — using cash at the cafeteria, using pen and paper and hand-delivering video on tape. Employees have been told it could be weeks before the systems are back up and running.

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