What, you've never heard of the grammer police?
I've seen nothing in the press saying he was a prohibited person. In fact, he bought the murder weapon from an FFL, so passed a background check.
Even if he's mentally ill, he's not prohibited from buying or posessing a firearm. Read the 4473 - it asks if the buyer has been
adjudicated mentally incompetent or
involuntarily committed to a mental facility. That's a pretty high standard.
Does AZ have tougher laws than the feds?
There's an interesting sub-topic - if state laws are more stringent, are they legal? I know states that look at things other than what's on the 4473 to determine CC licenses, but I don't know if they also apply to purchase or posession.
Then again, we're in the CA forum, and CA laws are blatantly unconstitutional yet lawmakers & enforcers haven't been punished. (Yet?)