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WASHINGTON—The Federal Bureau of Investigation is probing an apparently far-reaching penetration of data held by the Office of Personnel Management, in which the records of approximately 4 million individuals were compromised, according to people familiar with the matter.

U.S. officials suspect hackers based in China are behind the attack, though they continue to investigate, these people said. One official described it as one of the largest thefts of government data ever seen.

Investigators said the hack was a separate attack from one detected last year.

An FBI spokesman said the agency is working with other parts of the government to investigate. “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,” he said.

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