FBI Director James Comey says hackers who attacked Sony last month were sloppy, posting directly from a server used exclusively by North Korea.
Comey Wednesday urged the intelligence community to declassify information that proves the hackers used servers directly linked to the government in Pyongyang.
U.S. officials have previously blamed North Korea for the hack of Sony's corporate computers. Pyongyang has denied involvement, but praised the cyber attack.
The attack took place after North Korea warned Sony not to release a comedy film -- "The Interview" -- that depicted the fictional assassination of North Korean leader Kim Jong Un.
Comey made the comment Wednesday at a cyber security conference in New York. U.S. Director of National Intelligence James Clapper, who also spoke at the conference, called the cyber attack the the most serious ever against U.S. interests. He warned Pyongyang will continue online attacks against American interests unless Washington "pushes back."
Late last month, the FBI warned the hackers, who call themselves "Guardians of Peace," may target a leading U.S. news organization next.
U.S. theater chains were reluctant to show "The Interview" for fear of violence from the hackers. Sony delayed the scheduled opening, but then released the movie online and in a limited number of theaters.