The shooter was never committed.
That's where the hole in the law is for mentally ill people.
He reported for diagnosis, and he was referred to outpatient treatment. It's the same as someone with a chronic sleep disorder or an anxiety attack: considered a serious but low-grade manageable illness. He might have been OK if he had had regular follow-up talk therapy, but apparently not. The problem is, no one can compel someone to show up for that unless by court order, and being sick isn't a crime.
If he had been involuntarily committed, or if he had voluntarily committed himself following his diagnosis, then the law would have kicked in.