CENTENNIAL, Colo. (AP) — Investigators say the student who killed a classmate before taking his own life at a suburban Denver high school described himself in a diary as "a psychopath with a superiority complex" and indicated he was exacting revenge for being teased in elementary school.
Karl Pierson, 18, wrote that he planned the Dec. 13 attack at Arapahoe High School to start a conversation about elementary school teasing, according to investigative reports released Friday.
"Words hurt, can mold a sociopath, and will lead someone a decade later to kill," he wrote in a document titled "A diary of a madman."
Police have said Pierson held a grudge against his debate coach and was targeting him when he entered the school with a shotgun, a machete, homemade bombs and 125 rounds of ammunition. Pierson shot and killed 17-year-old classmate Claire Davis, who was sitting with a friend in a hallway, before taking his own life in the school library as security officers closed in on him.
The coach, identified in the reports as Tracy Murphy, escaped unharmed.
The shootings shocked Littleton Public Schools, one of many across Colorado that bolstered protocols for identifying the severity of threats and fashioning response plans after the 1999 shooting at nearby Columbine High School left 13 people dead. The two gunmen, both Columbine students, then killed themselves.
School officials deemed Pierson not a danger after a staffer overheard him in a school parking lot threatening to kill Murphy on Sept. 3, after the coach removed him as captain of the debate team. A school resource officer who is also a sheriff's deputy took a report of the threat two days later, but the department took no action against Pierson because he hadn't committed a crime, Sheriff Dave Walcher said.
"That was left in the hands of the school district," Walcher said. Superintendent Scott Murphy would not say at Friday's news conference how school administrators concluded Pierson was not a high-level threat.
Tracy Murphy requested an assistant principal obtain surveillance footage that might have captured Pierson yelling the threat, but officials did not search for it until after it had been deleted.
District Attorney George Brauchler, who reviewed the report, said he found no criminal offense aside from those committed by Pierson.
Murphy said he was "haunted" by the look Pierson had given him the day he was demoted. He told the assistant principal he feared for his safety and considered resigning.
Pierson was not suspended and was allowed to return to class less than a week after the threat. His mother had taken him out of school for three days. Officials kept Pierson from attending speech and debate practices, and a school psychologist ruled him a "narcissist," according to the reports.
The threat assessment indicated that the teen would see a psychologist once a week to talk about how to manage his anger. Pierson wrote in his diary in October that a visit with the psychologist was a "massive waste of time" and he was considering going on a shooting rampage at the office where his mother took him for a psychiatric examination.
It was at least the second time Pierson had threatened a teacher. In April 2013 he was suspended for "going off" on a teacher about a poor grade on a math test, his mother, Barbara Pierson, told authorities. She declined to comment Friday night.
Two days before the shooting, Pierson was sent to the assistant principal's office again after pounding on a locked classroom door and yelling, according to the documents. He was sent home for the day.
As an 18-year-old, Pierson legally purchased his shotgun on Dec. 6, and he showed pictures of it to friends in the days before the shooting. But no other students were aware of his plans, which he detailed in his diary, Walcher said. The short-tempered Pierson did tell friends he would kill Murphy, but they didn't take him seriously, the reports say.
The shooting plot was "a 10-year subconscious project for me to exact revenge, not on the individuals who perpetrated wrong, but instead by those I believe have done me wrong," Pierson wrote in his diary. "I will do something I have wanted to do for a while -- mass murder and be in a place of power where I and I alone are judge, jury and executioner."
The browser history on Pierson's laptop included searches for "rampage" and "school massacre," according to the report. Investigators also found images related to the deadly school shootings at Columbine High School and at Sandy Hook Elementary School in Newtown, Connecticut. Deputies removed the book "Columbine: A True Crime Story" and a calendar with a countdown to Dec. 13 from the Pierson home.
Associated Press writers P. Solomon Banda and Thomas Peipert contributed to this report.
Follow Sadie Gurman on Twitter at http://twitter.com/sgurman