Just when the cops have almost cracked the case, the FBI swoops in and takes over.
So, the cops are this close to cracking the case and catching the sonsofb*tches responsible for it.
Then the FBI show up out of nowhere and take over and hinder the progress of the case. What with their "red tape" and "following the rules" and "doing things ethically" and their "stealing all the glory for themselves".
Films and TV almost always depict the FBI, in these situations, as smug, self centered, irredeemable *ssholes who are pathological gloryhounds who do everything to impede the cops situation. The cops are almost always portrayed as heroic underdogs who you want to see succeed and rub their victory in the FBI's face because f*ck the government!
In the rarest, rarest of cases (see "Tick, Tick..." and "Boom" in Castle) they will actually show the FBI in a less *sshole-ish light and may even allow them to redeem themselves by the end. That, or they'll just have their top agent kidnapped. Whichever floats the writer's boat.