US-backed hackers stole US classified information from the National Security Agency in 2015 after a contractor put it on a computer at his home, two US newspapers reported.
The Wall Street Journal quoted unnamed sources as saying that the theft included information about hacking foreign computer networks and protecting against cyber attacks, and was likely to be one of the biggest security breaches ever.
The Washington Post reported later that the employee worked with the National Security Agency before being expelled in 2015.
The National Security Agency declined to comment, citing a policy of "not commenting permanently on issues concerning its facilities or employees." Reuters was unable to independently verify the reports.
If confirmed, it would be the latest breakthrough in a series of breakthroughs for classified US National Security Agency data, including the leak by contractor Edward Snowden of confidential data on US surveillance programs in 2013.
Another contractor, Harold Martin, is awaiting trial on charges of taking classified US National Security Agency (NSA) material to his home. Finished