CENTENNIAL, Colo. - At a news briefing on Friday, authorities released their final report on the 2013 Arapahoe High School shooting investigation and stated that the gunman wrote about his planned rampage at the school in a personal diary on his computer.
According to investigators with the Arapahoe County Sheriff's Office, senior Karl Pierson, 18, walked into the school on Dec. 13 armed with a shotgun, a machete, homemade bombs and 125 rounds of ammunition.
Investigators said Pierson shot and killed 17-year-old classmate Claire Davis before taking his own life in the school library as security officers closed in on him.
However, in the report released Friday, authorities point out that there were concerns about Pierson's behavior months before the attack, including a week when he was kept home in 2013 as school officials conducted a threat assessment on him. Pierson was judged a low threat.
In addition, Pierson's mother took him to a behavioral health center for an additional evaluation and he was judged not to be a threat to himself or others.
But neither mental health professionals nor school officials knew his innermost thoughts, which he kept in a detailed diary on his computer.
The digital diary, recovered by investigators, showed that on Sept. 17, 2013, Pierson called himself a "psychopath with a superiority complex" and confessed that "I will do something I have wanted to for a while... Mass murder...and be in a place of power where I and I alone are judge, jury and executioner."
Investigators pointed out that the diary entries were clear signs that the teenager could have been a threat.
Five days later on Sept. 22, Pierson seems to articulate a motive in his diary when he wrote, "When I do commit my atrocities, I want the conversation to be about elementary school teasing. Words hurt, can mold a sociopath, and will lead someone a decade later to kill."
In October, Pierson seemed confident that no one would find out his plan when he wrote, "My mother took me for a psych evaluation. Let the records show I lied through my teeth through the test."
By Nov. 24, he wrote that he was fully committed to his plan.
"It's weird going through life knowing that in 19 days I am going to be dead," Pierson wrote. "That's the hardest part is not being able to tell anyone."
On Dec. 8, Pierson purchased his gun.
"I bought my Stevens 320. It was not the initial gun I was expecting, but I think it will work better. I like the pistol grip," he wrote.
By Dec. 12, Pierson wrote that he had what he needed. "I bought a sling, ammo belts, and of course, ammo. It included 5 sabat slugs! I think I'll need more."
The diary also contains a detailed plan for his entry and attack at the school. Pierson wrote that he envisioned a 20-minute homicidal rampage, assaulting specific areas of the school, tossing Molotov cocktails and reloading.
He made it clear he wanted to do his killing late in the semester during finals week, when most students would be in class.
School officials say there were follow-ups after the initial threat assessment, but the extent is unclear.
Safety experts say such additional reviews are critical in preventing violence.
"In the case of most school shooters, their deterioration of behavior has been an on-going process," said Kenneth Trump of National School Safety and Security Services. "A threat assessment has to be an on-going process. A one-time look at one threat is not adequate."