The hoaxer texted from a cell phone using unsecure means. He disconnected his phone number, but it won’t matter.
I’m not sure it won’t. AFAIK a properly managed “burner” phone could be untraceable to the actual owner. Same for an anonymous email account tied to it.
I’m not saying they (FBI) won’t be able to tie it back, just that if someone knows how they can create an anonymous identity that (AFAIK) can’t be tied back.
And you certainly shouldn’t trust the FBI (or any other law enforcement or intelligence agency) on such matters. They tend(based on public statements) to be incredibly optimistic about their abilities, and AFAIK the ones who really understand the limits (of their abilities) aren’t allowed to speak to the public.