Sony is still struggling to restore some systems on Tuesday, eight days after a big cyber-attack, as investigators combed for clues to identify the cause of the breach.

Some employees at Sony's entertainment unit were given new computers to replace old ones that had been attacked with the rare data-wiping virus. The virus made their machines "unable to operate," a person with knowledge said to Sony's operations.

Studio co-chiefs Michael Lynton and Amy Pascal said in a company memo that "a large amount of confidential Sony Pictures Entertainment data has been stolen by the cyber attackers, including personnel information and business documents."

They added that they're not "sure of the full scope of information that the attackers have or might release," according to the memo, first reported by Variety. The memo encouraged employees to take advantage of identity protection services being offered.

Experts believe the breach is the first major attack on a U.S. company to use a class of malicious software designed to make computer networks unable to operate.

The FBI confirmed on Tuesday that it is working with Sony's home country of Japan on the investigation.

The hack, which was launched Nov. 24, affected computers with Microsoft Corp's Windows software, meaning Sony employees using Apple Macs had not been affected, according to the person familiar with Sony's operations, who spoke with Reuters.

Sony Pictures Entertainment shut down its internal computer network last week in order to prevent the data-wiping software from causing more damage.

People claiming responsibility for the attack have posted a number of digital copies of yet-to-be-released Sony films and what they claim are sensitive data about its operations and employees on Internet download sites.

For example, Sony's upcoming holiday movie "Annie", which is due to be released Dec. 19 in the United States, was available for download on a piracy site on Tuesday night.

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