Moonlight Maze, the first APT, was active in the late 90s and seemed to have disappeared at the turn of the century. The group and its attacks achieved mythical status in the cyber-security world and were the subject of many books. Investigators collected mountains of evidence but failed to make any headway into the investigation. Investigations stalled and eventually stopped, as Moonlight Maze disappeared from the face of the Earth during the early 2000s. Researchers were still detecting their attacks, but they didn’t know it was this mythical group from the 90s.
Source: https://www.bleepingcomputer.com/news/security/21-years-later-experts-connect-the-dots-on-one-of-the-first-cyber-espionage-groups/