Hackers aligned with the Russian government penetrated NSA computer systems and stole U.S. cyber secrets.

The theft occurred in 2015 after a contractor put information on his home computer.

Unidentified sources told U.S. media that the theft involved information about how to penetrate foreign computer networks and protecting against cyber attacks. Experts believe it may be one of the most significant security breaches thus far.

One report said the contracted worker had been employed at the NSA’s Tailored Access Operations unit for elite hackers before his employment was terminated the same year the breach occurred.

If confirmed, the hack would mark the latest in a series of breaches of classified data from the secretive intelligence agency, including the 2013 leaks of data on classified U.S. surveillance programs by contractor Edward Snowden.

Some U.S. lawmakers are critical of the NSA, saying the agency doesn’t seem to be serious about protecting its secrets.

“The NSA needs to get its head out of the sand and solve its contractor problem,“ Sen Ben Sasse, R-Neb., said. ”Russia is a clear adversary in cyberspace and we can’t afford these self-inflicted injuries.”

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