WASHINGTON (AP) — 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 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 spokesman for the Chinese Embassy in Washington called such accusations "not responsible and counterproductive."

A U.S. official, who declined to be named because he was not authorized to publicly discuss the data breach, said it could potentially affect every federal agency. One key question is whether intelligence agency employee information was stolen. Former government employees are affected as well.

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. The agency said it is offering credit monitoring and identity theft insurance for 18 months to individuals potentially affected.