All that news article says is that ONE AR-15 was found in the shooters car and (apparently) not used in the shooting and that (4) handguns were found inside the school. This says nothing about another rifle not being used to kill victims.
you'll have to re-watch the video from the 28 second mark forward. if no rifle was brought into the school, I have serious doubts a loaded one was laying around for him to pick up and use it on the kids.
The consolidation of the States into one vast empire, sure to be aggressive abroad and despotic at home, will be the certain precursor of ruin which has overwhelmed all that preceded it. --General Robert E Lee