Authorities said Tuesday that during an autopsy, the coroner's office found writing on the arm of the suspect, 18-year-old Karl Pierson.
"The shooter had used an indelible marker to write a phrase in Latin on the inside of his forearm. The Latin phrase is 'Alea iacta est,' which translates to English as 'The die Has Been Cast,'" the Arapahoe County Sheriff's Office said in a statement.
Also scrawled on his arm were five letters and numbers, which investigators believe corresponded to the locations of the Arapahoe High School library and four adjacent classrooms. It has not yet been determined whether Pierson had specific targets in mind in those areas.
The sheriff's office says Pierson walked through the north entrance of the high school on Friday afternoon with a shotgun, three Molotov cocktails, a machete and more than 125 rounds of ammunition. He immediately opened fire randomly inside the school, shooting 17-year-old Claire Davis in the head while she was sitting near the library. She remains hospitalized in critical condition.
Pierson's motive is believed to be revenge against the debate team instructor, who was also the school's librarian, after he was kicked off the team. The librarian escaped, and Pierson killed himself in the library 80 seconds after entering the school.
ABC News and The Associated Press contributed to this report.