The hack-attacks occurred last December but were not detected till April, the US government's Office of Personnel Management (OPM) said on Thursday in a communique, adding that it is working with the FBI to investigate the data breach.
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The OPM will send notices to approximately four million people whose information could have been compromised, said the agency, which is the human resources department for the federal government, conducting more than 90 percent of federal background and security-clearance investigations.
The US officials told The Washington Post that they suspect the attacks were mounted by Chinese info-pirates.
The cyberattack could be the largest theft of federally-archived personnel data ever perpetrated against the US.