Sony Hack Was Inside Job ... Investigators Claim

The Sony Pictures hack was masterminded by a former Sony employee -- at least partially -- according to a security firm which is now refuting the FBI's claim that North Korea was responsible.

Cybersecurity firm Norse Corp says its identified six suspects directly involved in the attack on Sony's records -- and they live in Canada, Thailand, Singapore ... but the former employee is based in the U.S.

According to the Daily Caller ... Norse Corp says the ex-staffer -- a woman identified as "Lena" -- has "extensive knowledge of the company's network and operations" and was laid off in May.

The information flies in the face of the FBI ...which firmly pointed the finger at Kim Jong-un last week.

Important to note ... Norse Corp was not hired by Sony and was conducting its own independent probe into the hack. We've contacted Sony to see if they're now working with Norse Corp ... so far no word back.