A report has said that the United States knew all along that North Korea was behind the hack attack on Sony Pictures as it had secretly breached the country's computer networks in 2010.
According to The New York Times, hidden software had alerted the U.S. intelligence officials to North Korean hacking activity. The report cited U.S. officials and leaked documents from the National Security Agency (NSA).
The U.S. Office of the Director of National Intelligence said that the nation was "fully aware" of North Korean efforts to infiltrate U.S. commercial networks and was tracking them routinely.
Intelligence officials believed that the hackers spent over two months preparing for the attack by building a map of Sony's systems.
North Korea has consistently denied any involvement in the cyber attack.
Several sensitive documents including, salary details and confidential emails between executives, were leaked during the attack.
It also caused Sony's film, The Interview, a comedy about an assassination attempt on North Korean leader Kim Jong-un, to be shelved briefly before being released online.
