If they can involve other people on some internet forum, that would be the claimed federal nexus

[Ocelot II]

that would make federal prosecution possible (see #7) - using the internet (interstate commerce) to plot with coconspirators. So far it looks pretty thin, but even though the outcome would be the same - the death penalty regardless whether he's convicted by Utah or the feds - Patel and Bondi would surely love to claim his head for themselves.