I am a lawyer, and you’re correct. It’s typically called a “jurisdictional hook.” In certain regards the US Constitution limits the U.S. Congress’s ability to legislate to issues that touch on “interstate commerce.” If it all happens within Montana’s borders, that’s typically for Montana to handle in its own state legislature. So, for the DOJ to investigate something, they have to then satisfy whatever jurisdictional hook that the Congress put in the law, which in turn makes the law constitutional (in the sense of: within the Congress’s jurisdiction to legislate about). Here, the Congress will have required a proof of “use of interstate wires” (or something to that effect).