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According to Reuters, The National Intelligence Service (NIS) said in a statement that it's possible that North Korea orchestrated the attacks, with the same allegations appearing in the South Korean media. Since Internet access is strictly controlled in North Korea, it's hard to imagine a mere group of government sympathizers conducting such an attack, which, according to NIS, requires "meticulous preparations."
If a North Korean government-approved cyber attack squad was really behind the attack, it's an interesting case of cyber warfare on a state level, something we've never seen before. Having hackers secretly attacking a site is one thing; having such an organization with considerable means at their disposal is another.