US cybersecurity officials were convinced North Korea was behind the notorious Sony hack last November because the NSA had secretly infiltrated the hermit kingdoms computer systems years before the Hollywood e-mail raid, according to a new report.

The National Security Agency penetrated North Korean networks in 2010 over concerns the nations digital infrastructure was considered one of the most impenetrable targets on earth, The New York Times reports.

The NSAs classified program placed malware that could trace the workings of North Korean hackers and followed a secretive system that traveled from Chinese and Malaysian networks back into a North Korean intelligence service.

Evidence gathered during the US cyber-surveillance mission convinced President Obama that hackers backed by the North Korean government were responsible for the Sony attacks, the paper said.

The hackers released embarrassing personal e-mails from Sony Pictures bigwigs in an attempt to thwart the release of a movie that lampooned North Korean despot Kim Jong-un.

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NSA hacked North Korea computers in 2010