The SF-86 breach could have dire consequences for U.S. intelligence gathering, former officials said, noting that it would make it extremely difficult for anyone inside the database to ever work in a covert capacity. For example, that would include someone employed by the State or Agriculture departments who gathers intelligence for the Defense Intelligence Agency.

“This is not the end of American human intelligence, but it’s a significant blow,” Brenner said.

As of October, 4.5 million Americans were cleared for access to classified information, including approximately a million contractors.

And because the SF-86s are stored in an indexed database, that database could also be combed for secrets, said Robert Caruso, a former Navy special security officer who has worked in security at the State and Defense departments. For example, Chinese agents could search the database for instances when agents with NSA covers were in the same place at the same time and make reasonable deductions about what they were doing there.