Russian government-backed hackers stole highly classified U.S. cyber secrets in 2015 from the National Security Agency after a contractor put information on his home computer, two newspapers reported on Thursday.

As reported first by The Wall Street Journal, citing unidentified sources, the theft included information on penetrating foreign computer networks and protecting against cyber attacks and is likely to be viewed as one of the most significant security breaches to date.

In a later story, The Washington Post said the employee had worked at the NSA’s Tailored Access Operations unit for elite hackers before he was fired in 2015.