According to the FBI’s statement, “As a result of our investigation, and in close collaboration with other U.S. government departments and agencies, the FBI now has enough information to conclude that the North Korean government is responsible for these actions.” This is based on evidence found in the code of the malware, which apparently are similar and have been used in previous North Korean attacks in the past.
A report from CNN also revealed that the attacks were routed through a number of other countries, such as China, and the FBI was able to unmask the IP addresses thanks to the help of the NSA. Other than that, not much else is known about the hack, suggesting that perhaps the rest of the report could be confidential.
North Korea has since denied that they are involved in the Sony Pictures hack, and there have been several reports that suggest that this might actually not be an attack from North Korea, and that North Korea might simply be used as a scapegoat, but either way hopefully we will get more details soon.