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Hackers breached the computers of the US government agency that collects personnel information for federal workers in a massive cyber attack that compromised the data of about 4 million current and former employees, US officials said on Thursday.
A US law enforcement source told Reuters a foreign entity or government was believed to be behind the cyber intrusion against the Office of Personnel Management (OPM), and media reports said authorities suspected it originated in China.
The Federal Bureau of Investigation said it had launched a probe and would hold the culprits accountable.
OPM detected new malicious activity affecting its information systems in April and the Department of Homeland Security (DHS) said it concluded at the beginning of May that the agency's data had been compromised.
The breach affected OPM's IT systems and its data stored at the Department of the Interior's data center, a shared service center for federal agencies, a DHS official said on condition of anonymity.
OPM had previously been the victim of another cyberattack, as have various federal government computer systems at the State Department, the U.S. Postal Service and the White House.
"The FBI is working with our interagency partners to investigate this matter," the bureau said in a statement. "We take all potential threats to public and private sector systems seriously, and will continue to investigate and hold accountable those who pose a threat in cyberspace."
