He also didn't have his gun(s), backpack or duffel bag with him.
He ditched them in a 3rd floor stairwell and then blended in with the students exiting the scene.
He even had time in between to get a drink at a Subway outlet in the nearby Walmart.
If wearing a mask how did they know he was the shooter, to within minutes arrest him? body build? This is not making any sense.