19 January 2015
Last updated at 12:47
North Korea has denied being behind the attack on Sony Pictures.
The US knew North Korea was behind the Sony Pictures hack because it had secretly infiltrated the country’s computer networks in 2010, according to the New York Times and Der Spiegel.
The newspapers cited US officials and leaked documents from the National Security Agency.
The New York Times said hidden software had alerted US intelligence services to North Korean hacking activity.
North Korea has consistently denied involvement in the security breach.
American investigators believe the hackers spent two months building up a map of Sony’s systems before the hack took place, the papers say.
November’s attack on the company saw the leak of sensitive documents including salary details and confidential emails between executives.
It also resulted in Sony film The Interview, a comedy about an assassination attempt on North Korean leader Kim Jong-un, being briefly shelved and then released online.
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.
“I’m almost certain they were doing it long before 2010,” he told the BBC.
“North Korea has been a target for the US for quite some time.”
Dr Murdoch said that if the NSA had been aware of the hack before it had happened, it may have chosen not to warn Sony for its own security reasons.
“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,” he added.
“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.”