The US government on Thursday admitted hackers may have accessed the personal data of four million current and former federal employees, in a cyber-attack suspected to have originated in China.
"As a result of the incident," uncovered in April, the Office of Personnel Management said it "will send notifications to approximately four million individuals."
The office handles hundreds of thousands of security clearances and background investigations on prospective government employees each year.
It was not immediately clear whether the hack affected senior government officials.
The Washington Post quoted government officials as saying that Chinese hackers were behind the breach.
The FBI and Department of Homeland Security are said to be carrying out an investigation.
Officials refused to assign attribution or motive, but pointed affected parties to measures that could prevent fraud and identify theft.
"Protecting our Federal employee data from malicious cyber incidents is of the highest priority," Office of Personnel Management director Katherine Archuleta said.
Government staff will be notified from June 8 if they are affected.