The US would most certainly deny any involvement in this shutdown....as Cyber attacks are looking more and more like the wave of future warfare. In the case of North Korea, 99% of its internet access is reserved for its government and military. Shutting them down would be as big a problem for N. Korea, as attacking our electrical grid would be for us.
The days of bombs and bullets are quickly becoming secondary...in comparison to the damage that can be done to a populace or government via a computer. Our government and Industries would be well advised to invest heavily in IT security.