AP - Hackers broke into the U.S. government personnel office and stole identifying information of at least 4 million federal workers.
The Department of Homeland Security said in a statement Thursday that at the beginning of May, data from the Office of Personnel Management and the Interior Department was compromised.
"The FBI is conducting an investigation to identify how and why this occurred," the statement said.
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 compromised.
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.
In November, a former Department of Homeland Security contractor disclosed another cyberbreach that compromised the private files of more than 25,000 DHS workers and thousands of other federal employees.
DHS said its intrusion detection system, known as EINSTEIN, which screens federal Internet traffic to identify potential cyber threats, identified the hack of OPM's systems and the Interior Department's data center, which is shared by other federal agencies.
"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 statement said.