Hackers recently penetrated a federal database and stole personal information on 4 million current and former federal employees in what officials say is the largest ever breach of a U.S. government network. The infiltrators also could have stolen information about U.S. operating procedures and people who never worked for the government.

“This is the most significant breach of federal networks in U.S. history," House Homeland Security Committee Chairman Michael McCaul, R-Texas, said in an interview with CBS.

The breach likely occurred in December 2014, but was not discovered until April. Only late last week, however, did the Obama Administration make public the breach at the Office of Personnel Management. Some, including McCaul, accuse China of being behind the attack. No definitive word on the identity of the perpetrators has been established, but an investigation is underway. “It was either a state actor or a group of very sophisticated hackers who probably worked in concert with a state actor,” said Rep. Adam Schiff (D-Calif.)

Schiff added that the information the hackers stole was “very valuable.” An anonymous American official told ABC last week that, in addition to data on federal employees, people who never worked for the government might have had information about them stolen, too. This information could have been taken from SF-86 forms, which are used in federal employees’ background checks. "If the SF-86's associated with this hack were, in their entirety, part of the stolen information, then that would mean the potential release of a staggering amount of information, affecting an exponential amount of people," an anonymous American official told ABC.

Administration officials have said that the Office of Personnel Management will be sending out notifications to people whose data may have been taken as part of the breach over the next two weeks.