HACKERS who targeted Sony Pictures over the release of the film The Interview "got sloppy" and inadvertently revealed their links to North Korea, according to the director of the FBI.
Speaking at the International Conference on Cyber Security James Comey said hackers had mistakenly sent messages that could be traced to IP addresses used exclusively by North Korea.
Mr Comey said the North Korean origins of the cyber attack were evident despite the use of proxy servers in other countries to throw investigators off their trail.
"It was a mistake by them," he said. "It made it very clear who was doing this."
The US federal investigations chief added that he had a "very high confidence" that the attack was carried out by North Korea, "as does the entire intelligence community".
Mr Comey would not disclose any evidence to support his claim, arguing that "because it will happen again we have to preserve our methods and sources".
"[Critics] don't have the facts that I have, don't see what I see," he said.
North Korea denies it was behind the attack. Many Western experts have also cast doubt on claims the country was involved.
Yesterday the US director of national intelligence James Clapper warned that North Korea would continue the attacks against American interests unless the United States "pushes back" at the country's targets.
North Korean people "really do believe they are under siege from all directions," and "are deadly, deadly serious about affronts to the supreme leader," he argued.
North Korean servers suffered a series of attacks that took the country's entire internet offline for more than nine hours, with service interruptions continuing for days.
The timing of the internet blackout led to speculation that the United States had caused it; the White House and the State Department declined to comment.
Sony Pictures initially postponed the release of the film The Interview after a breach of its cyber security and threats against cinemas. It was eventually released on its originally planned date without incident.
The film depicts a fictional plot to assassinate North Korean head of state Kim Jong Un.
In response to the alleged attack by North Korea, the US has also imposed new sanctions on three organisations and 10 individuals with links to the North Korean government and military.