The FBI has posted a press release describing why they think it's North Korea. While there may be more things we don't know, on its face it's complete nonsense. It sounds like they've decided on a conclusion and are trying to make the evidence fit. They don't use straight forward language, but confusing weasel words, like saying "North Korea actors" instead of simply "North Korea". They don't give details.
The reason it's nonsense is that the hacker underground shares code. They share everything: tools, techniques, exploits, owned-systems, botnets, and infrastructure. Different groups even share members. It is implausible that North Korea would develop it's own malware from scratch.
Here's the thing with computer evidence: you don't need to keep it secret. It wouldn't harm Sony and wouldn't harm the investigation. It would help anti-virus and security vendors develop signatures to stop it. It would crowd source analysis, to see who it really points to. We don't need to take the FBI's word for it, we should be able to see the evidence ourselves. In other words, instead of saying "IP addresses associated with North Korea", then can tell us what those IP addresses are, like "188.8.131.52".
But the FBI won't do that. They aren't in the business of protection but control. The idea that Americans should protect themselves and decide for themselves is anathema to the FBI.