"Especially since the shooter used an AR-15 assault rifle."

[Siiigh] No, George-he didn't. 

You can't buy assault rifles, and He used a Sig Sauer MCX carbine, a smaller caliber rifle that has NO major parts that interface with AR-15s in any way, shape or form.

Hey! Guess who can educate you about this topic?  The National Rifle Association!  Hey! Guess what every Islamic terrorist and mass shooter has in common? NONE of them are NRA members!

If You are blaming anyone or anything OTHER than freedom-hating Muslim Islamists and their sympathizers for this, you're wrong. Nothing and no one else has this blood on their hands.