FBI Director James Comey said on Wednesday that hackers behind the cyberattack on Sony Pictures Entertainment provided key clues to their identity by sometimes posting material from IP addresses used exclusively by the North Korean government.

The hackers, who called themselves "Guardians of Peace," sometimes "got sloppy" and failed to use proxy servers that would hide their identity, Comey said at the International Conference on Cyber Security in New York.

"The Guardians of Peace would send emails threatening Sony employees and post online various statements explaining their work. In nearly every case they would use proxy servers in sending those emails and posting those statements," Comey said.