Agreed that #1 is most likely. In fact, it seems probable that if this hack were indeed carried out by NK, the NSA is likely to have known about it during the planning or execution stages. Certainly they have eyes and ears on NK officials. While they wouldn't step in to prevent the attack (not their job), certainly they are active with the FBI to make decisions on the source and technical nature of the attack.
I wouldn't expect the FBI to do a thorough job on matters of digital security or cryptography, but I certainly would expect that of NSA. And therefore it seems unlikely we will be told all the evidence that they have collected in order to conclude NK is responsible.
Edit: This assumes they are telling the truth, which they might not be. But if they do honestly think NK is behind this, they certainly have a wealth of secret evidence supporting that decision.
I wouldn't expect the FBI to do a thorough job on matters of digital security or cryptography, but I certainly would expect that of NSA. And therefore it seems unlikely we will be told all the evidence that they have collected in order to conclude NK is responsible.
Edit: This assumes they are telling the truth, which they might not be. But if they do honestly think NK is behind this, they certainly have a wealth of secret evidence supporting that decision.