LOS ANGELES -- Cybersecurity experts are questioning the FBI's claim that North Korea is responsible for the hack that crippled Sony Pictures. Kurt Stammberger, a senior vice president with cybersecurity firm Norse, told CBS News his company has data that doubts some of the FBI's findings.

"Sony was not just hacked, this is a company that was essentially nuked from the inside," said Stammberger.

Kurt Stammberger, a senior vice president with cybersecurity firm Norse CBS News

While Norse is not involved in the Sony case, it has done its own investigation.

"We are very confident that this was not an attack master-minded by North Korea and that insiders were key to the implementation of one of the most devastating attacks in history," said Stammberger.

He says Norse data is pointing towards a woman who calls herself "Lena" and claims to be connected with the so-called "Guardians of Peace" hacking group. Norse believes it's identified this woman as someone who worked at Sony in Los Angeles for ten years until leaving the company this past May.

"This woman was in precisely the right position and had the deep technical background she would need to locate the specific servers that were compromised," Stammberger told me.

Other experts in cybersecurity and private intelligence are also questioning the FBI's claim that North Korea is solely to blame for the Sony hack.

"There are certainly North Korean fingerprints on this but when we run all those leads to ground they turn out to be decoys or red herrings," said Stammberger.

For instance while the malware used to attack Sony has been used by North Korea before, it is also used by hackers around the world every day.

It's worth noting that the original demand of the hackers was for money from Sony in exchange for not releasing embarrassing information. There was no mention of the movie "The Interview," which will see a limited theatrical release on Christmas Day. The FBI is still continuing its investigation into the Sony hack.