How did North Korea pull off the Sony hacking?

(CNN) - Exactly how does the U.S. know North Korea is behind the Sony hacking?

For nearly a month now, the FBI has been back-tracking data released online, working to identify who put it there. Sources tell CNN that hackers stole the credentials of a Sony administrator, then went to town, lurking around the system for months and stealing information. And at times, the FBI even thought other countries like Iran and China might have been involved.

So what led U.S. intelligence to this point?

CNN chief national security correspondent Jim Sciutto reports for "The Lead."