The Department of Homeland Security said in a statement that data from the Office of Personnel Management and the Interior Department had been compromised.
The Office of Personnel Management is the human resources department for the federal government, and it conducts background checks for security clearances. The OPM conducts more than 90% of federal background investigations.
The agency said it is offering credit monitoring and identity theft insurance for 18 months to individuals potentially affected. The National Treasury Employees Union, which represents workers in 31 federal agencies, said it is encouraging members to sign up for the monitoring as soon as possible.
Cyber-security experts also noted that the OPM was targeted in 2014 in a cyber-attack that was suspected of originating in China. In that case, authorities reported no personal information was stolen.
One expert said it is possible that hackers could use information from government personnel files for financial gain. In a recent case disclosed by the IRS, hackers appear to have obtained tax return information by posing as taxpayers, using personal information gleaned from previous commercial breaches.
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