Natalie Weinstein/CNET

FBI chief James Comey says that hackers who targeted Sony's networks over film "The Interview" were sloppy in their methods -- and the US Director of National Intelligence doesn't think North Korea has much of a sense of humor.

Speaking at an event Wednesday at Fordham University in New York, the bureau's chief said the hackers used proxy servers in an attempt to disguise their identity but "several times they got sloppy."

"Either because they forgot or because of a technical problem, they connected directly and we could see that the IPs they were using...were exclusively used by the North Koreans," Comey said, as according to cybersecurity news website Dark Reading. "They shut it off very quickly once they saw the mistake, but not before we saw where it was coming from."

The FBI says it has little doubt North Korea was behind the cyberattack due to these mistakes. According to the New York Times, Corney said these sloppy methods allowed the cyberattack to be traced directly back to North Korean addresses.

Corney acknowledged that decoys were used in an effort to wipe away traces of their identity, but did not specify particular mistakes made which has given the FBI "high confidence" in unmasking North Korea as the culprit.

Several weeks ago, President Obama took an unusual step in naming the country as culpable and has slapped sanctions on North Korea as a result.

Obama said the sanctions were necessary as a response to the "provocative, destabilizing, and repressive actions and policies of the government of North Korea, including its destructive, coercive cyber-related actions during November and December 2014."

Director of National Intelligence James Clapper also spoke at the event, said North Koreans "really do believe they are under siege from all directions," and "painting us as an enemy that is about to invade their country every day is one of the chief propaganda elements that's held North Korea together."

When it comes down to the film which caused all the fuss in the first place, a comedy surrounding two journalists enlisted by the CIA to assassinate North Korean leader Kim Jong-un, Clapper said he's seen it and "it's obvious to me that North Koreans don't have a sense of humor."

In related news, a white paper released by South Korea's Defense Ministry on December 6 estimates that North Korea's "cyberarmy" -- focused on digital warfare -- has grown in number to 6,000. As reported by Ars Technica, the report warns that North Korea has also made advances in terms of nuclear warfare.

This story originally posted as "FBI chief: North Korean Sony hackers 'got sloppy'" on ZDNet.