Were the AR15s the muslims used modified in any way?
Yep!
They weren't CA compliant. Supposed to have 'bullet button' on the mag catch to make in inconvenient to remove the magazine. The mag is allowed to be more than 10 rd (lots of loaded 30 rd recovered), muzzle brake must be pinned or welded, stock must be fully extended and pinned to prevent collapse. None of which matters SQUAT. Pulling the trigger when aimed at defenseless humans was the actual crime.