I'm baffled, too, I don't think CPD did their best work that day. I hope we hear more about that at some point.
I grew up in Arizona. I'm not surprised, not because CPD screwed up that day but because I don't think you can truly understand the situation if you don't live in Arizona, Texas, and a few other states like Montana, Wyoming, and possibly Idaho.
Even before "stand your ground" laws, the legal environment was very much influenced by the old west heritage. I learned it was perfectly legal to kill someone defending your livestock, which could include chickens, cattle, goats, etc. I think the law has changed and I'm still trying to find out if this was really true. As someone said, gunfighting in the streets was legal in some areas in Arizona until 1967. Ranchers would sometimes shoot at trespassers. That mostly stopped in the late 70s when a rancher was aquitted of killing some illegal immigrants but was retried under Federal law and convicted of interferring with interstate commerce (Hobbs Act). See
United States v. Hanigan, 681 F.2d 1127, 1130-31 (9th Cir. 1982)
In these days of horrible school shootings I'm sure things have changed but I would sometimes take a rifle or shotgun to school so that I could go hunting on the way home. This was not against school rules. You just could not have the gun loaded on school property. You did not even have to declare you had it. You just had to take it directly from your car to your locker and back at the end of the day. Guns were not allowed on the bus. You also had to have a hunting license and, at the time, this required passing a firearms safety class if you were below a certain age.
I'm in my 50s and when I was in high school around 35-40 years ago, you occasionally saw people walking around town openly carrying a gun on their hip. They generally had good reason because they were jewelers or others who often had valuables on their person but still, it is a huge change from today. At that time concealed carry was illegal but open carry was perfectly legal. That kind of open carry was pretty rare in Tucson and Phoenix even back then but not at all uncommon in smaller towns.
So a shooting under the circumstances represented by Alex, Lori, and Tylee that morning would not seem too suspicious at the time. This was not a gang shooting or a robbery. This was by all appearances a case of self defense. Yes, I believe it was a setup. But based on the evidence police had at the time it was not suspicious on the surface. I think as they continued to investigate there were more things that looked out of place. But AC died before they could take any action.