In 1992, Congress passed a law saying everything was to be declassified in 25 years. October 2017 came and went, and the FBI blocked around 7,500 documents from release. The DEA didn't want CI names related to organized crime in them. The State Department blocked some due to "national security and foreign affairs concerns".
Trump claimed while running for President that he was now going to release everything, yet the FBI and CIA stepped in again and those plans changed.
Trump finally said he couldn't release them due to some being "of such gravity that it outweighs the public interest in immediate disclosure".
So they remain secret, 60 years later.