FBI Director James Comey.

NEW YORK — FBI Director James Comey said this week 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.

“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 it”, Comey said.

“We could see that the IP addresses they used … were IPs that were exclusively used by the North Koreans. It was a mistake by them. It was a very clear indication of who was doing this. They would shut it off very quickly once they realized the mistake, but not before we saw them and knew where it was coming from,” he added.

Sony’s network was crippled by hackers in November as the company prepared to release “The Interview”, a comedy about a fictional plot to assassinate North Korean leader Kim Jong Un. The attack was followed by online leaks of unreleased movies and emails that caused embarrassment to executives and Hollywood personalities.

Comey urged the U.S. intelligence community to declassify information that showed the hackers used such servers. Critics of the FBI and spy agencies have accused the government of failing to back up assertions that North Korea was responsible.

Comey said investigators still do not know how hackers got into Sony’s systems. But he said technical analysis of the malware used showed strong similarities to malware developed byNorth Korea and used last year in attacks on South Korean banks.