The Federal Bureau of Investigation (FBI) insists North Korea was behind the Sony cyberattack and claims it has the digital smoking gun to prove it.

In a cybersecurity conference in Manhattan on Wednesday, FBI top sleuth James Comey said  North Korean hackers "got sloppy" and failed to cover their digital tracks when it sneaked into Sony Pictures Entertainment portals.

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Comey said the hackers known as Guardians of Peace made use of proxy servers to disguise the real origin of the emails and statements. However, there were times when they got sloppy, he added.

Several times, the North Koreans connected directly and shut off the connection right away after discovering the mistake But the investigators already beat them to it and found out where the emails were coming from.

Last month, the FBI blamed North Korea for the hack that forced Sony to stop the showing of its still unreleased film "The Interview", a comedy about a wacky plot to assassinate North Korean dictator Kim Jung Un that starred Seth Rogen and James Franco.

The FBI said it based its assumptions on strong similarities to malware used last year by North Korea in attacking financial institutions in South Korea. But critics accused the bureau of finger pointing and not having enough evidence to tie the North Koreans to the deed. Some suggested the usual suspects, like Russia.

Comey urged the U.S. intelligence community to declassify information to show that the hackers indeed used the IP addresses they now have.

 "I know that some serious folks have suggested we have it wrong," Comey said. "They don't have the facts that I have. They don't see what I see...I have very high confidence about this attribution."

U.S. Director of National Intelligence James Clapper, who also spoke at the same cybersecurity conference, said the Sony cyberattack was the worst ever against U.S. interests. He accused his opposite number in North Korean intelligence Gen. Kim Youn Chol of the Reconnaissance General Bureau as "responsible with the overseeing of the attack against Sony."

TagsFBI, James Comey, North Korea, Cyberattack, James Clapper, Sony Hack