[palmer]

Can a FBI agent initiate an investigation for any federal crime on their own

Doubtful, otherwise there would be chaos. Also it doesn't change my basic point that there are very few people in a position to investgate FISA crimes.

[shotgun]

Check out this link. Pretty good information about how they start an investigation:

https://www.brennancenter.org/analysis/just-what-fbi-investigation-fact-sheet