4 Million US Government Workers Hit by Cyber Breach

The Federal Bureau of Investigation said on Thursday that it has launched a probe and would hold the culprits accountable, Reuters reported.

"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."

The Office of Personnel Management (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 office handles employee records and security clearances.

"This would likely be the largest theft of US government data in the history of the United States," Patty Culhane, Al Jazeera's White House correspondent reporting from Washington, said.

"Basically, OPM is like the human resources department of the entire federal government. They also get security background checks for people who want to get security clearances," she said.

"The big question remains exactly what information was stolen? Was it social security number, your federal ID or was it salary information. Right now OPM is not saying."

A US law enforcement source told Reuters a "foreign entity or government" was believed to be behind the cyberattack. Authorities were looking into a possible Chinese connection, a source close to the matter said.

A Chinese embassy spokesman in Washington said hypothetical accusations were irresponsible and counterproductive.

"Jumping to conclusions and making [a] hypothetical accusation is not responsible," and is "counterproductive", Chinese embassy spokesman Zhu Haiquan said in emailed comments.

The OPM had previously been the victim of a cyberattack, as have various federal government computer systems at the state department, the US Postal Service and the White House.