There's also an independent investigation called "Operation Blockbuster", detailed results here: https://www.operationblockbuster.com/wp-content/uploads/2016...

That report does not specifically attribute the Sony hacks to a particular nation-state. It does demonstrate the hack was not the result of a disgruntled employee, but rather an APT group the report refers to as "The Lazarus Group."

In the interests of full disclosure, I worked with the people involved in that operation.