DENVER - The Arapahoe High School gunman planned to target at least five areas of the school -- the library and four other classrooms, according to a sheriff's report released Tuesday.
The shooter had written on his arm in permanent marker five separate letters/numbers -- numbers which correlate directly with the identification of the library and specific classrooms located in the immediate area of the Library/Media Center of the Arapahoe High School, the sheriff said.
The teenage gunman also had the Latin phrase "Alea iacta est," on the inside of his forearm, which translates to English as "The die has been cast."
The gunman, identified as 18-year-old Karl Pierson, was armed with a 12-gauge, pump-action shotgun when he entered the school via the north entrance on Friday afternoon. He made no effort to conceal the weapon from the view of others, Arapahoe County Sheriff Grayson Robinson said.
In addition to the shotgun, the shooter also brought three functional Molotov cocktails, a machete in a canvas scabbard and more than 125 rounds of assorted types of (steel-shot, buckshot and slug) shotgun ammunition into the school, the sheriff said.
The shooter carried the numerous rounds of shotgun ammunition in two bandoliers that were worn across his chest and waist, while the remaining items were carried in a backpack he wore, Robinson said.
Upon entering the school at 12:33 p.m. Pierson immediately and randomly fired the shotgun and then fired a second time, causing serious injury to 17-year-old senior Claire Davis.
The shooter fired the weapon three additional times, without causing injury to students, faculty or staff.
Even though he had plans for further terror, the gunman never made it out of the library.
Within 80 seconds after entering the school, Pierson realized that the school's resource officer was approaching and Pierson shot and killed himself in the library, Robinson said.
Robinson has credited the SRO, other responding deputies, and the active shooter protocol for saving many lives.
"I believe that their quick response and their reaction saved lives in this particular incident," Robinson has said. "I believe the shooter knew that deputy sheriff's were immediately about to engage him and I believe that shooter took his life because he knew that he had been found."
Investigators discovered the writing on Pierson's arm while examining his body, with the assistance of the Arapahoe County Coroner's Office.
"The letters/numbers written on the shooter's arm is an indicator to investigators of the shooter's plan," the sheriff's office said Tuesday.
Teams of sheriff's office investigators will attempt to determine the relevance of the list of numbers written on the shooter's arm to specific classrooms, while closely evaluating if the occupants of those classrooms may have also been targeted for harm by the shooter.
Although it currently appears that the shooter acted alone, the ongoing investigation will include consideration of any individual who may have provided any form of assistance regarding the tragic incident, the sheriff's office said.
"The robust and ongoing investigation into the Arapahoe High School shooting is being conducted in a deliberate and methodical manner. Additional investigative resources from the Colorado Bureau of Investigation and the Federal Bureau of Investigation are assisting with the vast responsibilities associated with the complex investigation," the sheriff's office said.
Davis remains in critical condition in a coma. She was shot in the head at point-blank range, the sheriff said.