Ok I think I see what happened and why everyone is confused. "Crimes against the United States" means any federal crime. It doesn't just mean like, terrorism or insurrection. It's not a crime against Uncle Sam or the President. It's anything prohibited by law.
The unauthorized access case that has been filed against Brian is "United States v. Brian Laundrie." Making a bank sad by using someone else's bank card is a crime against the United States.
Murder is a crime against the United States.
18 U.S. Code § 1111 - Murder
Manslaughter is a crime against the United States.
18 U.S. Code § 1112 - Manslaughter
Murdering someone on federal land is a federal crime that will be prosecuted by the feds.
The FBI does not lead investigations in Florida on behalf of a Wyoming county attorney. They lead federal investigations into federal crimes.