Update: Chinese Hackers Steal 4 Million US Personnel Records

China-based hackers are suspected of breaking into the computer networks of the U.S. government personnel office and stealing identifying information of at least 4 million federal workers, American officials said Thursday.

The Department of Homeland Security said in a statement that data from the Office of Personnel Management and the Interior Department had been compromised.

"The FBI is conducting an investigation to identify how and why this occurred," the statement said.

The hackers were believed to be based in China, said Sen. Susan Collins, a Maine Republican.

Collins, a member of the Senate intelligence committee, said the breach was "yet another indication of a foreign power probing successfully and focusing on what appears to be data that would identify people with security clearances."

A U.S. official who declined to be identified said the data breach could potentially affect every federal agency. One key question is whether intelligence agency employee information was stolen. Former government employees are affected as well.

"This is an attack against the nation," said Ken Ammon, chief strategy officer of Xceedium, who said the attack fit the pattern of those carried out by nation states for the purpose of espionage.

The information stolen could be used to impersonate or blackmail federal employees with access to sensitive information, he said.

The Office of Personnel Management is the human resources department for the federal government, and it conducts background checks for security clearances. The OPM conducts more than 90 percent of federal background investigations, according to its website.

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