The FBI says it is investigating threatening emails sent to some employees of Sony Pictures Entertainment, which was hit by a cyber-attack last week.
The attack disrupted the company's computer system and spewed confidential information onto the internet.
The FBI was trying to identify the person or group responsible, the agency said in a statement.
The trade paper Variety said the email written in broken English claimed to be from the head of the Guardians of Peace - the same group that took credit for last week's attack.
"Removing Sony Pictures on earth is a very tiny work for our group which is a worldwide organisation. And what we have done so far is only a small part of our further plan," the email read.
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Later, the email warns employees: "Please sign your name to object the false of the company at the email address below if you don't want to suffer damage. If you don't, not only you but your family will be in danger."
"Make your company behave wisely," the email urges.
The Los Angeles Times reported the email was received on Friday.
A Sony spokesman confirmed in an email to the Times that "some of our employees have received an email claiming to be from GOP" and said the company was working with law enforcement.
Employees were told to turn off their mobile devices after receiving the message, Variety said, citing Sony insiders it did not name.
Recently, hackers released personal information for thousands of Sony employees online, including some Social Security numbers and the purported salaries of top executives.
Five movies, including the unreleased Annie, also have shown up on file-sharing websites.
On November 24, workers who logged onto Sony Pictures' network saw a skeleton and the message "Hacked by #GOP".
