SAN FRANCISCO (CBS/AP) — The FBI has confirmed it is investigating a recent hacking attack at Sony Pictures Entertainment, which 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. 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, as well as the still-in-theaters “Fury,” although a direct connection to the hacking hasn’t been confirmed.

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Online security expert Gary Miliefsky believed the attack could be linked to the pending release of Sony’s comedy “The Interview” starring Seth Rogen and James Franco, in which the CIA sends two men to assassinate North Korean leader Kim Jong Un.

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“If I were Sony, I would be prepared, if they come out with the movie “The Interview” on December 25th, to expect stronger attacks from North Korea through their operatives around the globe.”

Culver City, California-based Sony Pictures said in a statement Monday that it is continuing “to work through issues related to what was clearly a cyber attack last week. The company has restored a number of important services to ensure ongoing business continuity and is working closely with law enforcement officials to investigate the matter.”

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Along with the FBI, Sony has brought in forensic experts from the Mandiant division of FireEye, a Silicon Valley cybersecurity company, according to a source familiar with the case who did not want to be named because the companies have not yet announced the arrangement. Mandiant helps companies determine the extent of breaches and repair damages. The firm has worked on other high-profile computer breaches, including the one at Target last year.