Mary McGregor via AP
This Sept. 7, 2013 photo shows Arapahoe High School senior Karl Pierson as he poses for his class picture in Centennial, Colo. Pierson shot fellow student Claire Davis at the school on Friday, Dec. 13, 2013, before killing himself.
The Colorado school-shooting gunman wrote the Latin phrase Alea iacta est — meaning "The die has been cast" — on his arm, along with numbers and letters that investigators believe outlined his plan for carnage, officials said Thursday.
Karl Pierson, 18, who was upset he lost his spot on the debate team, entered Arapahoe High School on Friday heavily armed and looking for the speech coach, who was also the librarian, friends and officials have said.
But police now suspect he may have had other targets because of the markings found on his forearm after he critically wounded senior Claire Davis and killed himself.
The numbers and letters he wrote next to the Latin phrase "correlate directly with the identification of the Library and specific classrooms located in the immediate area," the Arapahoe Sheriff's Office said in a statement.
"The letters/numbers written on the shooter's arm is an indicator to investigators of the shooter's plan," the statement continued, adding that investigators would will be "closely evaluating if the occupants of those classrooms may have also been targeted for harm by the shooter."
Although he wounded just one student, Pierson came to the school with a small arsenal: a .12 gauge pump-action shotgun, three Molotov cocktails, a machete in a canvas scabbard, and more than 125 rounds of ammunition in two bandoliers strapped across his chest and in his backpack.
In an attack that lasted just 80 seconds, he fired immediately upon entering and then fired again, shooting Davis, 17, in the head. Her family has said she is in critical but stable condition and comatose.
Pierson fired three more times without hitting anyone before killing himself inside the library, police said. The librarian, Tracy Murphy, had fled outside when he heard the gunman was looking for him — which police praised as a tactical move to lure the shooter away from students.
Ed Andrieski / AP file
The co-captain of the debate team, Joe Redmond, told NBC News that Pierson had an acrimonious relationship with Murphy that led to a "death threat." He also was a fan of the bomb-making manual "Anarchist's Cookbook," a revelation that led the author to forcefully call for it to be taken out of print.
“I did try to help him and didn’t get to him,” Redmond said of his efforts to talk to Pierson. “No one was able to get to him. At the end of the day, that’s what happened.”
Davis' family has asked that those who want to make a donation to Claire instead send money to local youth mental health awareness and support programs through the Littleton Hospital Foundation or to TLC Meals on Wheels, according to the hospital Facebook page.
In a statement issued Monday, Pierson's family said they cannot comprehend their son's actions.
"We are shattered by the tragic events that took place on Friday," Barbara and Mark Pierson said.
"Our thoughts and prayers are with Claire Davis and her family. They, and she, have suffered unimaginably, and we pray for her full recovery. We also pray for the entire Arapahoe High School community, as we know your lives are forever changed by this horrific event.
"As parents, we loved our son Karl dearly and we are devastated by what happened Friday. We cannot begin to understand why Karl did what he did. We ask for privacy during this unthinkably difficult time and hope that you will respect our need for time to grieve."
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This story was originally published on Tue Dec 17, 2013 3:59 PM EST