Updated

The teen gunman who fatally shot a fellow student at his suburban Denver high school earlier this month entered through a door that was propped open.

Arapahoe County Sheriff Grayson Robinson said Monday that the Arapahoe High School door is supposed to be locked but was frequently left open. If it had been locked, Robinson thinks 18-year-old Karl Pierson, armed with a shotgun, machete and over 100 rounds of ammunition would have just have found another way inside.

Pierson shot senior Claire Davis point-blank in the face soon after entering through the open door. Investigators believe his main target was a speech team coach who had disciplined him but also say he likely wanted to hurt many others. He killed himself as a deputy assigned to the school closed in.