Hackers broke into the computer network of the Department of Energy last month and stole personal data belonging to several hundred employees and contractors, the department confirmed Monday.
The department notified employees that their information was at risk in an email Friday and urged them to encrypt all files and emails containing sensitive information.
The email said "several hundred" employees were affected, but no classified data was compromised in the attack.
"As individual affected employees are identified, they will be notified and offered assistance on steps they can take to protect themselves from potential identity theft," the department told employees in an email forwarded by a department spokesperson to The Huffington Post.
The source of the attack, which occurred in mid-January, was still unknown. But Chinese hackers are "likely suspects because the department is known to be a major target of China for both secrets and technology," according to the Free Beacon, which first reported the attack.
Chinese officials have repeatedly denied any involvement in sponsoring hackers.
The attack against the Energy Department marks the latest revelation of hackers breaking into corporate or government computer systems. Last week, The New York Times, The Washington Post and The Wall Street Journal acknowledged their networks were compromised by hackers from China looking to find reporters' sources. On Friday, Twitter said it had been hacked by unknown attackers who gained access to passwords and other information for as many as 250,000 user accounts.