The only portal that N Korea has to the internet is through China. N Koreans are almost exclusively trained at Chinese Universities where past attacks have originated.
I put my money on the N Koreans operating out of China as this has happened many times before and the hackers are just sent home to N Korea.
China is very good at finding hackers and Chinese are too scared to do it, just ask the Chinese Google employees.
China Has numerous Cyber Guerilla Teams run through Universities and Private Companies this Falls Under the Military as their Cyber attack squad