An ongoing investigation has found that North Korea had sought to use proxy servers to conceal the Sony hack, FBI Director James Comey said at a cybersecurity conference in Manhattan.
But the hackers sometimes "got sloppy" and sent messages that could be traced to IP addresses used exclusively by the North Korean government, he said.
"I have very high confidence about this attribution to North Korea, as does the entire intelligence community," Comey said.
Comey said he was hesitant to reveal more about how US officials learned that North Korea was the source "because it will happen again, and we have to preserve our methods and sources."
Earlier today, Director of National Intelligence James Clapper said North Korea will continue the attacks against American interests unless the United States "pushes back.