"Under a 1992 law inspired by the conspiracist movie "JFK," the National Archives was supposed to have released all of the remaining records by midnight ET — unless President Donald Trump objected on national security grounds."
There was no "investigation", just a records release directive in 1992. So there wasn't anything new to protect as you seemed to believe.
apparently the latest release describes a plan to use bio and chemical weapons to overthrow fidel Castro and the Cuban Govt.