A newly disclosed document from the U.S. National Security Agency reveals that for at least 4 years, the agency has been infiltrating the North Korean computer networks with malware in order to keep track of every move done by North Korean hackers who are allegedly made up of around 6,000 people.

The investigators from the agency recently determined that the hackers gained access to the identity credentials of a Sony systems administrator. Eventually, the group was able to explore the company's network for more than two months which ended up with a series of leaks and downtime in Sony's computer-based operations.

The FBI on its part has released evidence that North Korea is behind the Sony attack, something that security experts had taken with some skepticism. However, the recent findings about the spying operations of NSA against North Korea had somehow supported the FBI's statement.

The findings also helped explain why Obama seemed to have no doubt when he strongly accused North Korea of instigating the cyber attacks and even deciding on placing the country under heavier sanctions.

"The speed and certainty with which the United States made its determinations about North Korea told you that something was different here - that they had some kind of inside view," said cyber warfare expert James A. Lewis from the Center for Strategic and International Studies in Washington. "Attributing where attacks come from is incredibly difficult and slow."     

While the U.S. intelligence agency had been able to tap into the computer networks of North Korea, they didn't find any signs of threat that could serve as a warning on the impending Sony attacks. It was the FBI who said that the malware used in the attacks contained Korean-language development tools.

According to FBI Director James Comey, the North Koreans are sloppy when it comes to hiding their tracks.

"We could see that the IP addresses that were being used to post and to send the emails were coming from IPs that were exclusively used by the North Koreans," said Comey.

Obama's accusation against North Korea for the Sony hack was described by Kim Jong Un as "groundless slander." In an attempt to clear the nation's involvement in the attack, the North Korean leader offered to extend its help in finding the perpetrators and that the U.S. should agree. Furthermore, North Korea had warned that should the U.S. refuse their proposal, it would definitely lead to serious consequences.