Assume the mob kidnaps the sheriff of a county of Colorado (18 USC 351 does not apply to the Sheriff, right?) and they never cross the state border. During the kidnapping, the mob's enforcers wound a deputy of the sheriff, but he survives (so 28 USC 540 doesn't apply, right?).
Who is responsible for investigating this: FBI, Colorado State Police, or the Sheriff's department?
Because kidnapping is a violation of state law, the Colorado Bureau of Investigation would have jurisdiction to investigate. Assuming the crime occurred in the sheriff's home county, the undersheriff would take over and could also investigate.
And if the sheriff is missing for more than 24 hours, federal law assumes that he was kidnapped using the instrumentalities of state commerce, giving the FBI jurisdiction to investigate, as well. Even before 24 hours, the fact that the mob acts as part of a criminal enterprise with an effect on interstate commerce, would also give the FBI jurisdiction to investigate under RICO.
The FBI doesn't sit around waiting for other agencies to prove beyond a reasonable doubt that the FBI has jurisdiction. The FBI is allowed to be proactive: they have every right to investigate whether a case falls in their jurisdiction. Of course, that involves the same sleuthing needed to solve the case itself.
As an example, the FBI did respond to the 2007 Boston Mooninite Invasion... They turned out not to be a crime in the FBI's jurisdiction, or even a crime of any size at all.
