(CNN) -- 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.
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."
CNN's Carma Hassan contributed to this report.