Arapahoe County Sheriff Grayson Robinson said Karl Pierson had written reminders on his arm of five classrooms and the library he planned to attack, the Denver Post reported Wednesday.
Pierson brought with him a 12-gauge pump shotgun, three Molotov cocktails and a machete, Robinson said. He also had 125 rounds of ammunition on two bandoleers.
On the inside of his foreman, Robinson said the 18-year-old had written in Latin: "Alea iacta est" – "The die has been cast."
"The shooter was intent on causing the maximum amount of harm," the sheriff said.
Pierson fired five times, with one of his shells hitting Claire Davis, 17, in the head last Friday. She remains in a coma in critical condition.
The shooter lit a firebomb in the library that set three bookshelves on fire. He then shot himself in the head.
Duggar sisters unveil Christian dating rules in new book
Ohio bar shooting arrested, charged with murder