The FBI was recently briefed on an alternative theory about who hacked Sony Pictures.
Norse, the cybersecurity intelligence firm who briefed the FBI, doesn’t believe North Korea is responsible, instead they believe that a disgruntled employee working with a hacking group perpetrated the attack.
The theory is supported by recent evidence that the hackers were communicating in Russian instead of Korean.
There have been a number of cyber security experts who have questioned the validity of the FBI’s claim that North Korea launched the attack. The LA Times and The Atlantic have more.