The papers detailing finding tunnels under the mcmartin preschool are not FBI papers but are taken from a book about the mcmartin trial by Ted Gunderson who is one of the primary proponents of the satanic ritual abuse theory. The investigation was funded by the families and none of the people involved are reputable. You can find another copy of the whole report here:

http://tedgunderson.info/index_htm_files/McMartin%20Scientif...

you will find the exact same pages as included in the finders fbi file. Even if you choose to believe the report, this demonstrates that it didn't actually have anything to do with the finders and in any case was not done for, or by, the fbi or police. it was (presumably) included in the file because it had similar claims to ones made against the finders.

I have read the entire finders fbi file. Repeatedly there are the fbi and police fielding claims that the finders were satanists, or that they were sexually abusing the children, but in every case they document they found no actual evidence of this.

the claim of an fbi cover-up was documented in the fbi file (that would be weird if you think about it) but this claim was made by a single junior member of the manhattan beach police, and is never repeated or corroborated anywhere.

the claim that the finders were tied up in the intelligence community seems to be based on uncorroborated claims as well. nothing in the document or linked resources supports it.

I hope you find this helpful, I had multiple of my friends bring up this fbi release as "proof" that SRA was real all along but after reading every page of the file I just don't think there's anything compelling in there.