Arapahoe County Sheriff Grayson Robinson said the Dec. 13 incident that killed one student and the gunman, could have been far deadlier were it not for a "very effective lockdown protocol" at the school.
The incident began when student Karl Pierson entered the school through a door that was supposed to be locked but had been left ajar. Pierson, 18, armed with a 12-gauge shotgun, hundreds of rounds of ammunition, a machete and three Molotov cocktails, shot fellow student Claire Davis at close range.
She survived for several days before dying at an area hospital.
As the school went into lockdown, Pierson made his way to the library where he was said to be looking for librarian Tracy Murphy, his debate team coach. Robinson said Pierson held a grudge over disciplinary issues but Murphy, acting on a warning that Pierson was targeting him in the rampage, fled the building, the Denver Post said.
Pierson reached the library, threw one of the Molotov cocktails at a bookshelf, then shot himself. He fired six rounds in all during the incident.
Robinson said the preliminary investigation has found Pierson obtained the shotgun legally. Other questions about his mental state and particulars of his planning have yet to be answered.
One thing is already clear, Robinson said: "There is no question that the individual who entered that school armed is a murderer."