It’s a complex issue what to do with such a teenager.
I’m all for civil liberties, but they belong to people who can distinguish between right and wrong and can distinguish between when, if you have such thoughts, you do not express them nor allow them to simmer. If he just returned a few days ago and now expresses this kind of thing, he damn well better go back. And I mean, for 6 months minimum.
It will be interesting to see what deeper investigation reveals. Wondering if his parent sent him to Maine so Vermont authorities wouldn’t know? Vermont has a terrible record of inputting mental defectives to NICS and it doesn’t appear Maine put him in the system either.
It should be like threatening the president. Interview and assess, if the threat is real then there are places to send them to get help or at least keep us safe.