Employees at Sony Pictures Entertainment received threatening emails on Friday that claimed to be from the group that carried out a massive cyber attack at the Hollywood film studio.
The email said it came from GOP, the shorthand for "Guardians of Peace," the group that claimed responsibility for the hacking of Sony that began Nov. 24, a Sony spokesman said.
The attack crippled the studio's computer network and exposed sensitive data. The identity of the Sony hackers has not been determined, and it was not clear if the emails came from the same group.
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Sony, a unit of Sony, did not provide a copy of the email or detail its contents.
The FBI, one of the U.S. government agencies probing the hacking, "is aware of threatening emails that have been received by some employees at Sony Pictures Entertainment," FBI spokesman Joshua Campbell said in a statement.
"We continue to investigate this matter in order to identify the person or group responsible," he said.
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The emails could be from copycats purporting to be the hackers who had obtained the addresses of Sony employees from the gigabytes of data leaked over the Internet, said Marc Maiffret, chief technology officer of cybersecurity firm BeyondTrust.
"Anybody could have written this," said Maiffret. "You are going to have a lot of people leveraging the stolen data that is available online for harassment and fraud."
The emails could mark the first high-profile follow-on attempt to harass the company by other parties. Fraudsters are likely to use other stolen data including Social Security numbers, salaries, mailing addresses and proprietary information about the company's operations to attempt to engage in a wide variety of scams for a long time, he added.
Sony has hired security firm FireEye and its Mandiant forensics unit to investigate the hacking.
North Korea is a principal suspect in the attack, a U.S. national security source told Reuters on Thursday. A North Korean diplomat denied that Pyongyang was behind the hack.
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North Korea had vehemently denounced Sony film "The Interview," a comedy about a plot to assassinate North Korean leader Kim Jong Un, scheduled for release on Dec. 25.