The agency, which is conducting background checks, warned it was urging potential victims to monitor their financial statements and get new credit reports.
U.S. officials believe this could be the biggest breach ever of the government's computer networks.
The breach is beyond the Office of Personnel Management and the Department of Interior, with nearly every federal government agency hit by the hackers, government officials said.
An assessment continues, and it is possible millions more government employees may be affected.
U.S. investigators: We believe this was China's work
U.S. investigators believe they can trace the breach to the Chinese government. Hackers working for the Chinese military are believed to be compiling a massive database of Americans, intelligence officials told CNN on Thursday night.
It is not clear what the purpose of the database is.
The Washington Post and Wall Street Journal first reported Thursday that Chinese hackers were responsible for the breach.
A spokesman from the Chinese Embassy in Washington objected late Friday to allegations that the Chinese government may be behind the massive hack.
"Cyberattacks conducted across countries are hard to track, and therefore the source of attacks is difficult to identify. Jumping to conclusions and making hypothetical accusation is not responsible and counterproductive," said Zhu Haiquan.
