“The FBI and U.S. intelligence agencies for years have been tracking the hackers who they believe to be behind the cyber attack on Sony, according to current and former American officials. And during that long pursuit, U.S. agencies accumulated still-classified information that helps tie the hackers to the recent Sony intrusion.
The Obama administration announced a round of sanctions against North Korea Friday, and explicitly said the measures were in retaliation for the ‘destructive and coercive cyber attack on Sony Pictures Entertainment.’”
(Source: U.S. Spies Say They Tracked ‘Sony Hackers’ For Years – The Daily Beast