My impression is that they found out the CIA was already involved
in some way and the FBI just closed the book to get out of their way.
The CIA's involvement could have been anything. Could have been an international cult they were looking into, an experiment to try and get kids to summon The Horned God, or a brainwashing experiment, or they could have been involved because some diplomat's kid was there and they wanted to influence them. My point is that without the CIA docs there's no way to tell what was really going on or how nefarious this really was.