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North Korea on Dec. 20 called on the U.S. to hold a joint investigation into the incident, after rejecting the conclusion by the FBI that it was behind the attack.

“North Korea would never admit it is responsible for the Sony hacking,” said Kim Jin Moo, a North Korea researcher at South Korea’s state-run Korea Institute for Defense Analyses in Seoul. “‘It can’t afford consequences like being put back on the list of state sponsors of terrorism. Yet it has to make sure its threat is taken seriously.”

North Korea has said it knows how to prove it had nothing to do with the hacking and proposed a joint investigation with the U.S.

Malicious software in the Sony attack revealed links to malware previously used by North Koreans, the FBI said. The tools used also were similar to a cyber-attack in March 2013 against South Korean banks and media organizations.

South Korea says North Korea operates a unit of elite cyber-hackers to disrupt enemy networks in the event of war and steal information from foreign computers. South Korea believes North Korea is behind at least six cyber-attacks it has suffered since 2009.

On Sunday, NDC repeated its denial that it was responsible for those attacks.

North Korea and the U.S., which fought each other in the 1950-53 Korean War, remain technically in a state of war because the conflict ended with an armistice, not a peace treaty. The U.S. stations about 28,500 troops in South Korea to deter aggression from North Korea.

The rivals are locked in an international standoff over the North’s nuclear and missile programs and its alleged human rights abuses. In the spring of last year, tension dramatically rose after North Korea issued a string of fiery threats to launch nuclear strikes against Washington and Seoul.