The senior entered Arapahoe High School Friday with a bandolier of ammunition strapped to his chest, and carrying a machete, three Molotov cocktails and a pump-action shotgun he used to shoot 17-year-old Claire Davis in the head, the Denver Post reported Saturday.
"His intent was evil, and his intent was to injure multiple people," Arapahoe County Sheriff Grayson Robinson said.
Police said Pierson bought the shotgun Dec. 6 with the intention of seeking revenge on the high school librarian and debate teacher, Tracy Murphy. Students said Murphy had disciplined Pierson in September over threats the 18-year-old made against the teacher.
While in the library looking for Murphy, Pierson fired his shotgun point-blank at Davis' head, though police don't believe he was specifically targeting the girl.
Davis was "an innocent victim of an evil act of violence," Robinson said. "There was no time for the victim to run from the shooter."
Davis was in critical condition Saturday night.