Washington, June 6 (RHC)-- The U.S. government has admitted hackers may have accessed the personal data of four million current and former federal employees. The Federal Bureau of Investigation, the FBI, said it had launched an investigation and would hold the culprits accountable who penetrated deep into the data held by the Office of Personnel Management (OPM).

The FBI said in a statement that "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 OPM said in a statement that it "detected a intrusion affecting its information technology (IT) systems and data" in April.

"The intrusion predated the adoption of the tougher security controls," OPM said. "Certainly, OPM is a high value target," said OPM Chief Information Officer Donna Seymour. "We have a lot of information about people, and that is something that our adversaries want."

The OPM, which conducts background checks for security clearances, declined to comment on who was behind the hack. Some U.S. officials, who spoke on conditions of anonymity, claimed that hackers from China are behind the attack, according to a Washington Post report.

One official familiar with the matter called it as one of the largest thefts of U.S. government data ever done.