So, Cooper sweeps into Twin Peaks to solve the mystery of who killed Laura Palmer... along the way, he also busts up a drug ring, and becomes involved in arresting an international criminal who shot him (Josie).
I mention these last two facts to demonstrate that Cooper does exercise his FBI authority on criminal matters unrelated to Laura's death.
Now, here's the big question: Why, after Cooper discovers that half the town was banging an underage girl, does he have Harry arrest any of these men? Ben Horne, Jacques Renault, Leo Johnson, not to mention what seemed to be most of the rest of the male populace of Twin Peaks, were all positively identifies as having had sex with a seventeen year old girl. Did Washington have some kind of different statutory rape statues, or was Cooper letting a bunch of guys off?
