So Who Shut Down North Korea's Internet?

Just this past Friday, North Korea's already shaky internet access started to crumble. Over the weekend, things just got worse, and by yesterday morning, the country was in a state of total blackout. Considering that the U.S. just officially blamed North Korea for the Sony hack, and that the U.S. asked China for help in bringing North Korea down, and that North Korea has shoddy internet access in the first place--who's to blame?