Anxiety heightened on the Sony Pictures lot after employees received a threatening email from someone claiming to be with the hacking group that infiltrated the studio's computer system last week.
The email, received on Friday, threatened several Sony employees and their families.
"It's false if you think this crisis will be over after some time," the email said, according to a copy obtained by Variety. "All hope will leave you and Sony Pictures will collapse. This situation is only due to Sony Pictures."
The email was allegedly written by "the head of the GOP." Previously, the group has identified itself as the "Guardians of Peace."
A Sony representative acknowledged the email.
"We understand that some of our employees have received an email claiming to be from GOP," a Sony spokesman said in an email to the Los Angeles Times. "We are aware of the situation and are working with law enforcement."
It was not immediately clear which Sony employees received the message. Not all employees were targeted.
The FBI also acknowledged the email.
"The FBI is aware of threatening emails that have been received by some employees at Sony Pictures Entertainment," the agency said in a statement. "We continue to investigate this matter in order to identify the person or group responsible for the recent attack on the Sony Pictures network. Recent events underscore the persistence and maliciousness of harmful cyber criminals, and the FBI will continue to identify and apprehend those who pose a threat in cyberspace."
The email was written in broken English. It asked employees to sign a statement disassociating themselves with Sony. "If you don't, not only you but your family will be in danger," the message said.
The email was the latest blow to hit the company as it scrambles to repair the damage of last week's massive computer breach.
Several times, the hackers released a hoard of documents on the Internet, some of which contained Sony employees' personal information, such as their salaries and social security numbers.
The hacking incident has been costly. Five of the studio's feature films have shown up on file-sharing sites, including some that had not yet been released, such as the musical "Annie," which is headed to theaters in time for Christmas.
The studio continues to investigate the hacking and has enlisted the help of Maniant, a cyberforensics unit of the security firm FireEye Inc.
Some experts have speculated that North Korea is responsible in retaliation for the upcoming Sony movie "The Interview," a comedy that depicts a fictional assassination attempt on leader Kim Jong Un.
Others have theorized that the attack could be the work of disgruntled current or former employees.
Times staff writer Meg James contributed to this report.