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Re:FBI Director: Sony’s ‘Sloppy’ North Korean Hackers Reveal... by roywiggins on Wednesday January 07, 2015 @03:56PM (#8853253)
I assume they're this confident because the NSA is snaffling up every 1 and 0 that comes in and out of NK. It would explain both their confidence and reticence to explain why.
Re:FBI Director: Sony’s ‘Sloppy’ North Korean Hackers Reveal... by esmi on Wednesday January 07, 2015 @04:08PM (#8853313)
There are also nontechnical ways to attribute the attack to North Korea. For example a human informant inside North Korea which they did not want to reveal could provide quite solid information.

But it appears we will never know the source of their confidence.