Hackers breached the US government's Office of Personnel Management (OPM) in December and stole the personal information of four million federal employees, officials said Thursday.
The FBI and Department of Homeland Security’s Computer Emergency Readiness Team is investigating the breach.
"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," the FBI said.
The OPM said in statement: "Within the last year, the OPM has undertaken an aggressive effort to update its cybersecurity posture, adding numerous tools and capabilities to its networks.
"As a result, in April 2015, OPM detected a cyber-intrusion affecting its information technology (IT) systems and data. The intrusion predated the adoption of the tougher security controls."
One official described the breach as one of the largest thefts of government data ever seen.
It is not known who is responsible, but officials told Reuters that 'a foreign government or entity is to blame'.