Actually the Feds cannot prevent that. They lost their case where they contended Horuchi could not be tried for murder, but the new district atty in Idaho declined to prosecute.
Or was "persuaded" not to prosecute, on pain of losing federal funding/support, or perhaps finding himself a target of a federal probe. Notice that the old prosecutor, who dared to bring the case, LOST his job before the case could be brought to trial.
Can anybody here think of a cse where a federal LEO was successfully put in jail for violating a private citizen's civil rights in the course of doing his job?