My parents live in a small Georgia town where the police force consists of a chief, a couple of superannuated officers, three squad cars and a paddy wagon. They would need help to handle something like this, and they would probably ask for it.
Also, in an emergency the old adage, "It's better to seek forgiveness than permission" probably applies.
It is unthinkable that a post Provost Marshal would (1) even think about doing this and (2) do it without the Post Cdr’s permission. It is unthinkable that a Post Commander would contenance this action without approval from his/her higher HQ. And so on.
If this was a good buddy kind of thing the US Army officer who authorized it should be court martialed yesterday.
If a governor accepted US Army law enforcement assistance he/she has no idea why he/she is commander-in-chief of the National Guard, is just stupid or some combination of the above. Last I heard Alabama was a red state. There is no way a Pubbie governor would sanction the US Army swerving into his lane unless a calamity beyond imagining occured. Although the multiple killings were tragic they were not calamitous.
It will be interesting to see how this plays out—assuming it even happened. I read a few days back that there are stories out there about US Army soldiers being involved in mysterious road blocks or whatever that had nothing to do with a military exercise per se. Perhaps, this just more grist for that mill.