Hackers made off with the personal data of 4 million of federal workers after breaking into the systems of the U.S. Office of Personnel Management and the Interior Department, the Department of Homeland Security (DHS) announced Thursday.

The cyber breach occurred in early May and is now being investigated by the Federal Bureau of Investigation. The identifying information of at least 4 million federal workers was exposed, but one U.S. official told the Associated Press that the breach could potentially affect every corner of the federal government. That's because the Office of Personnel Management is essentially the human resources branch of the government and is responsible for conducting the vast majority of the federal background checks on government employees.

Multiple U.S. officials are blaming China for the breach. Sen. Susan Collins, R-Maine, 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," the AP reported.

"DHS is continuing to monitor federal networks for any suspicious activity and is working aggressively with the affected agencies to conduct investigative analysis to assess the extent of this alleged intrusion," the agency said in a statement.

Homeland Security said it became aware of the breach after EINSTEIN, its intrusion detection system, identified the hack. It remains unclear whether information for intelligence agency employees was also compromised, but it is feared that the stolen information could be used to either blackmail federal employees or used to impersonate them to steal sensitive information. 

In a statement, the FBI said, “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.”