Exactly, and that is the reason why I don't trust the US. Why not be 100% transparent in the investigation? What is there to lose?
NK hacks Sony. US uses technique x to attribute the attack to NK. In the course of a shared investigation NK learns what technique x is. NK changes their future operations to counter technique x. The US loses the ability to use technique x to attribute attacks.
I wish I knew too, and Im not trying to convince you to blindly trust them (because I don't either), but you have to at least acknowledge the fact that they may very well have "something to lose".
My wife was in the Navy for six years and had a relatively high security clearance for an enlisted member. More than once she knew far more about a situation than was being reported on the news.
Let's assume that the Fed is telling the truth and that they do in fact have hard evidence. As they have not presented this evidence, we have to conclude that it is classified. So, they cannot present the raw intel, but they also cannot step back and do nothing as this is a hot, national issue which is now about more than a security breach at a movie company.
So, they come forward with the accusation and assure the public that a "proportional and appropriate" response will be dealt.
Of course, this scenario paints the best possible picture of the Fed. It may or may not be the case. However, short of evidence which in no way compromises a classified source, it would be their only option.
I just don't see the motive behind framing NK for this. Given, I/we don't typically have a clue about what actually goes on at this level of international relations, but there is no obvious motive at present time.
And to be clear, I don't think they've sunk so low that "the FBI is framing NK" is anything but a garden variety conspiracy theory at this point, but "the FBI is rushing a very complex analysis under heavy political pressure" is plausible to me, as is "the FBI has strong evidence that they are not willing to show us".
Ultimately, assuming they do have classified evidence, there's also a cost-benefit consideration to be made - it doesn't have to be a given that they can't release it. I don't think this describes the present situation, but depending on the scale of the response a situation calls for, compromising one or two classified methods or sources in order to get public support for the appropriate action may be the lesser of two evils.