Those details were released Friday in an investigative report by the Arapahoe County Sheriff's Office. Pierson stated in the diary "words hurt, can mold a sociopath, and will lead someone a decade later to kill." Pierson wrote he planned the Dec. 13 attack at the high school to start a conversation about elementary school teasing.
In a Sept. 17, 2013 entry, Pierson wrote, "I will shoot up my school, Arapahoe High School, before the year is over." Only two pages of the 28-page diary have been released.
Arapahoe County Sheriff Dave Walcher says the attack was long planned. “It appears from looking at this case, clearly, that Karl Pierson was angry, in fact, very angry.”
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. The coach escaped unharmed.
Police have said Pierson planned to harm many people. He wrote numbers and letters corresponding to the library and four other classrooms on his forearm before entering the school.
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 in Jefferson County left 13 people dead. The two gunmen, both Columbine students, then killed themselves.
School disciplinary records obtained by The Associated Press showed in September 2013, Arapahoe High officials deemed Pierson "not a high-level of threat" after he shouted a death threat against the debate coach. That was after Pierson was demoted as debate team captain.
Pierson was allowed to return to class less than a week after his threat. The disciplinary records said Pierson showed no remorse over the incident.
Walcher says a threat assessment by his office and another by a behavorial health center also concluded Pierson was a low level of concern.
Littleton Public Schools Superintendent Scott Murphy attended Friday's news conference but declined to answer questions about the actions of staff members or students.
Two days before the shooting, Pierson was sent to the assistant principal's office after pounding on a locked classroom door and yelling, disturbing other classrooms, according to the documents. He was sent home for the day.
Authorities have said Pierson entered the school through a door that should have been locked but was propped open.
Christina Erbacher-Kolk, a school security guard at the time, has said administrators took no action after she and another guard saw Pierson looking up guns on his computer at school less than two months before the shooting. School officials refused to comment on Erbacher-Kolk's statements.