Police: Arapahoe HS shooter detailed "revenge" plot in diary

CENTENNIAL, Colo. - Karl Pierson, the 18-year-old who fatally shot a classmate and killed himself at a Colorado high school last December,wrote in a diary of "a 10-year subconscious effort to exact revenge," authorities said in a news conference Friday, where they announced the release of a 40-page report on the shooting.

"I will shoot up my school -- Arapahoe High School -- by the end of the year," Pierson wrote in a Sept. 17, 2013 diary entry, according to Arapahoe County Sheriff Dave Walcher.

The 28-page diary was recovered by authorities from a computer in the Pierson home.

"Karl Pierson was angry, and in fact, very angry. The anger goes back a long time," Walcher said Friday.

Police have said Pierson held a grudge against his debate coach and was targeting him when he entered the the suburban Denver school on Dec. 13, 2013, with a shotgun, a machete, homemade bombs and 125 rounds of ammunition. 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.

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 that left 13 people dead. The Columbine gunmen, both students there, then killed themselves.

School disciplinary records obtained by The Associated Press showed that in September 2013, Arapahoe High officials deemed Pierson "not a high-level of threat" after he shouted a death threat against the debate coach after he 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.

Littleton Public Schools officials have refused to discuss their handling of that incident but authorities say the investigation of it revealed school administrators did not act swiftly despite being aware of escalating concerns about Pierson.

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.

Christina Erbacher-Kolk, a school security guard at the time, has said that 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.

Authorities have said on the day of the attack, Pierson entered the school through a door that should have been locked but was propped open.

District Attorney George Brauchler said Friday that based on the information obtained by his office and authorities, he doesn't anticipate any criminal charges against mental health experts involved in the case or the school district.

Sheriff Walcher said the investigation found no one else was involved in Pierson's plot.