That number was originally thought to be about 1.1 million, OPM said in a statement. About 21.5 million individuals had their Social Security Numbers and other sensitive information affected by the hack.

The Office of Personnel Management announced Wednesday that 5.6 million people are now estimated to have had their fingerprint information stolen.

According to OPM, "federal experts believe that, as of now, the ability to misuse fingerprint data is limited." The office acknowledged, however, that future technologies could take advantage of this information.

An interagency working group—to include FBI, Department of Homeland Security and Defense Department representatives—plans to review the implications of the stolen fingerprint data, according to OPM.

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U.S. officials have privately blamed the breach on Chinese government hackers, but they have avoided saying so publicly. For its part, Beijing has denied any involvement.

Officials also have said no evidence has surfaced yet suggesting the stolen data has been abused, though they fear the theft could present counterintelligence problems.

—Reuters contributed to this report.