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).

I'm less certain of the following but as I understand it: if the event in question involves telephone/fiber lines or airwaves, it's usually a shoe-in for Fed jurisdiction anyway because those are generally regulated by some Fed agency and/or travel at some point through Fed-owned, regulated, or operated assets.

I'm now wondering if it's possible to make a within-state-only messaging service. What would something look like that manages to avoid Federal jurisdiction as much as possible?

Funny you should mention that. Look up the Texas power grid. Completely independent from the rest of the U.S. and has stayed that way despite some rather impressive drama w.r.t. the Midnight Connection Controversy.

You’d have to take into consideration Federal lands within states where federal laws apply: National Parks, military installations, and other Federal buildings like courthouses.

U.S. jurisdiction is complicated.

If your service sends signals from cellphones (radio waves), those are regulated by the FCC...

Fed jurisdiction extends in weird ways.