Story highlights Gunman had written plans to attack at least five areas of the school on his arm

He had also written the Latin phrase for "The Die Has Been Cast"

According to police, Karl Pierson shot 17-year-old Claire Esther Davis once in the head

Colorado shooter Karl Pierson had written plans to attack at least five areas of Arapahoe High School on his arm, as well as the Latin phrase for "The Die Has Been Cast," the Arapahoe County Sheriff's Office said Tuesday.

Specifically, he'd written five letters/ numbers on his arm, corresponding with the library and classrooms near it, that investigators believe indicate what he planned to do.

"Teams of Sheriff's Office investigators will attempt to determine the relevance of the list of numbers written on the shooter's arm to specific classrooms, while closely evaluating if the occupants of those classrooms may have also been targeted for harm by the shooter," the Sheriff's Office said in a statement.

According to police accounts, Friday's shooting began at 12:33 p.m. when Pierson, 18, walked into Arapahoe High School armed with a pump-action shotgun, a machete and a backpack containing three Molotov cocktails, a bandolier of ammunition across his chest.

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Photos: Arapahoe High School shooting Photos: Arapahoe High School shooting Arapahoe High School shooting – Claire Davis, a 17-year-old senior at Arapahoe High School, died Saturday, December 21, eight days after she was shot by a student with a shotgun at the school in Centennial, Colorado. "Despite the best efforts of our physicians and nursing staff, and Claire's fighting spirit, her injuries were too severe," Littleton Adventist Hospital said in a statement. Hide Caption 1 of 18 Photos: Arapahoe High School shooting Arapahoe High School shooting – Arapahoe County Sheriff Grayson Robinson holds a picture of Davis at a press conference on Saturday, December 14, after identifying her as the victim of the shooting at Arapahoe High School the day before. Hide Caption 2 of 18 Photos: Arapahoe High School shooting Arapahoe High School shooting – Robinson identified the shooter as 18-year-old Karl Halverson Pierson, who killed himself after opening fire. Pierson had asked about a specific teacher when he entered the school armed with a shotgun on December 13. Hide Caption 3 of 18 Photos: Arapahoe High School shooting Arapahoe High School shooting – Students are escorted out of Arapahoe High School in Centennial, Colorado, on December 13. Hide Caption 4 of 18 Photos: Arapahoe High School shooting Arapahoe High School shooting – Students wait outside Arapahoe High school after being evacuated on December 13. Hide Caption 5 of 18 Photos: Arapahoe High School shooting Arapahoe High School shooting – Officers escort students out of Arapahoe High School. Hide Caption 6 of 18 Photos: Arapahoe High School shooting Arapahoe High School shooting – Susie Ohle hugs Rob Escue as he came out of the school on Friday afternoon. Hide Caption 7 of 18 Photos: Arapahoe High School shooting Arapahoe High School shooting – Ruben Allen hugs his son Alex Allen, 17, after being evacuated from the school Hide Caption 8 of 18 Photos: Arapahoe High School shooting Arapahoe High School shooting – Students and parents walk away from the school. Hide Caption 9 of 18 Photos: Arapahoe High School shooting Arapahoe High School shooting – Law enforcement personnel arrive at the high school in a military-style vehicle. Hide Caption 10 of 18 Photos: Arapahoe High School shooting Arapahoe High School shooting – SaSha Meiler is overcome with emotion after hearing the voice of her younger sister, a freshman. Hide Caption 11 of 18 Photos: Arapahoe High School shooting Arapahoe High School shooting – Parents Cathy Thorson, left, and Heather Moran, facing the camera, embrace while they wait for news on their children. Hide Caption 12 of 18 Photos: Arapahoe High School shooting Arapahoe High School shooting – Students walk away from the school in single file with their hands up. Hide Caption 13 of 18 Photos: Arapahoe High School shooting Arapahoe High School shooting – Students from Arapahoe High School gather at the school's track. Hide Caption 14 of 18 Photos: Arapahoe High School shooting Arapahoe High School shooting – People gather outside the school. Hide Caption 15 of 18 Photos: Arapahoe High School shooting Arapahoe High School shooting – Parents wait for word about their children after a gunman opened fire at the school. Hide Caption 16 of 18 Photos: Arapahoe High School shooting Arapahoe High School shooting – Members of law enforcement are seen outside the school. Hide Caption 17 of 18 Photos: Arapahoe High School shooting Arapahoe High School shooting – Law enforcement personnel gather near the high school. Hide Caption 18 of 18

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He shot 17-year-old Claire Esther Davis once in the head amid shots fired randomly into school hallways. She is in critical condition.

He also ignited one of the firebombs in the school library before killing himself in the back corner of the library.

Police say Pierson was looking for revenge against a librarian but couldn't find the faculty member and killed himself as a school resource officer closed in.

Davis was just a random victim, police surmised, and the gunman and victim didn't know each other.

Pierson's parents released a statement Monday afternoon, offering prayers for Davis' full recovery.

"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," Barbara and Mark Pierson said. They also offered prayers "for the entire Arapahoe High School community, as we know your lives are forever changed by this horrific event."