A US law enforcement source said 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 US government has long raised concerns about cyber spying and theft emanating from China and has urged Beijing to do more to curb the problem. China, however, has denied US accusations.

The Federal Bureau of Investigation (FBI) 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 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 centre, which is a shared service centre for federal agencies, a DHS official said on condition of anonymity. The official would not comment on whether other agencies' data had been affected.
               
The OPM had previously been the victim of another cyber attack, as have various federal government computer systems at the State Department, the US Postal Service and the White House.

Since the intrusion, OPM said it had implemented additional security precautions for its networks. It said it would notify the four million people affected and offer credit monitoring and identity theft services to the people affected.

There was no immediate comment from the White House on the latest cyber attack.

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