Apparently the FBI referral order for mental health treatment is known as a 5150, and people who have that order turn up in a background check are not permitted to buy guns. I don’t know if it affects them in any other way - probably not, although they should certainly be on a no-fly list.
He had probably gotten his gun before the episode with the FBI, which I think was only in November, so the order didn’t have any effect in his case. But we have very lax procedures governing the mentally ill who threaten violence, especially ideologically motivated violence. Dylann Roof was also known to be nuts, for example, but people just tolerated it and assured themselves that he “didn’t really mean it.”
The other thing that is possible in the case of Esteban is that ISIS really had picked up on him, knew he was psychotic, and actually was urging him to do a little bit of jihad for them.
Reports are that he was turned over to the local pd