FBI Director James Comey said on Wednesday (January 7) he is confident North Korea was behind the cyber hack into Sony Pictures Entertainment because the group sometimes posted material from servers used exclusively by the North Korean government.

Comey was speaking at the International Conference on Cyber Security in New York. In the face of continued questioning about whether the U.S. was right in blaming North Korea for the Sony attack, he said he urged the intelligence community to declassify information that shows the hackers used servers linked to North Korea.

"There're a couple of things that I have urged the intelligence community to declassify that I want to tell you right now. The 'Guardians of Peace' would send e-mails threatening Sony employees and would post online various statements explaining their work. In nearly every case they used proxy servers to disguise where they were coming from, in sending those e-mails and in sending and pasting and posting those statements. But several times they got sloppy, several times either because they forgot or they had a technical problem, they connected directly and we could see them. And we could see that the IP (Internet Protocol) addresses that were being used to post and to send the e-mails were coming from IPs that were exclusively used by the North Koreans," Comey explained.