US Hacked North Korea Computers in 2012
According to reports, the US knew North Korea was behind the recent Sony Pictures hack, because it had infiltrated the country's computer networks back in 2010.
The New York Times said hidden software had alerted US intelligence services to North Korean hacking activity.
Since the Sony hack, North Korea has consistently denied involvement in the security breach.
American investigators believe the hackers spent months building up a map of Sony's systems before the hack took place.
The attack on Sony back in November saw the leak of sensitive documents, including salary details and confidential emails between executives, and also resulted in the film The Interview - a comedy about an assassination attempt on North Korean leader Kim Jong-Un - being briefly shelved and then released online.
According to reports, the US officer of the Director of National Intelligence and the Intelligence community was fully aware of North Korean attempts to infiltrate US commercial networks, tracking them routinely.
Spokesman Brian Hale said: "While no two situations are the same, it is our shared goal to prevent bad actors from exploiting, disrupting or damaging US commercial networks and cyber infrastructure.
"When it becomes clear that cyber criminals have the ability and intent to do damage, we work cooperatively to defend networks."
Cyber security expert Dr Steven Murdoch, from University College London said it was likely that the NSA had at least tried to access North Korean networks before.
He said: "I'm almost certain they were doing it long before 2010. North Korea has been a target for the US for quite some time."
Dr Murdoch added that if the NSA had been aware of the hack before it happened then it may have chosen not to warn Sony for its own security reasons.
He added: "One possibility is that they didn't know how damaging the attack was going to be, and didn't want to risk revealing their sources by mentioning it to Sony.
"Or maybe they did know [how harmful it was] but it wasn't that damaging by intelligence community priorities - this was very damaging to Sony but in terms of national security it's not as significant."
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