“It was already illegal for Cruz to buy a gun due to his documented past.”
What documented past? He bought the gun legally.
Yes - too past his previous events were not documented. I’m guessing that one of the 39 visits from the police might have led to an arrest if they pushed harder. A neighbor said he shot (or maybe shot at) their chickens. Regardless - that would be a crime - probably deserving of an arrest which may have come up for a weapons purchase.
Or the several times he was disciplined by the school - those might have risen to calling the police and an arrest, but the school kept it in-house to keep their records cleaner for more federal funding.
Not much use in all of the weapon laws with respect to convictions, etc. if nobody will charge the criminal.