Before he stepped foot into Arapahoe High School on Friday, Karl Pierson scrawled a Latin message on the inside of his forearm in permanent ink. Translation: “The die has been cast.”

Also written on his arm were numbers and letters associated with five classrooms and the school’s library, places authorities said Pierson planned to attack during his noon-hour rampage, Arapahoe County Sheriff Grayson Robinson said Tuesday.

The sheriff offered new glimpses into what he described as Pierson’s plan to harm many.

“And I believe it was a plan,” Robinson said, adding that the numbers and letters on the gunman’s arm were “to remind him where he wanted to go.”

Before killing himself inside the library, Pierson, 18, shot and critically wounded fellow senior Claire Davis, 17, who on Tuesday remained hospitalized in a coma.

The sheriff has said Pierson was targeting his debate coach and librarian, Tracy Murphy, when he came to school armed with a 12-gauge pump-action shotgun, three Molotov cocktails and a machete concealed in a canvas scabbard.

But those weapons, along with more than 125 rounds of assorted ammunition he wore in a pair of bandoliers strapped to his chest and waist, are a sign he wanted to wound more than just Murphy, Robinson said. Pierson packed a mix of steel-shot, buckshot and slugs, but the sheriff would not say which of those he fired.

“The shooter was intent on causing the maximum amount of harm,” Robinson said. He would not elaborate on what other evidence had been gathered by investigators, but he said they still had a great deal of work ahead of them. The FBI is helping with the “methodical” and “complex” probe.

Their goals include determining the relevance of the classroom numbers and whether anyone inside them was also a target.

As for the Latin message on Pierson’s arm, “Alea iacta est,” Robinson said “it pretty much speaks for itself.” The sentence is a line attributed to Julius Caesar, who said the words as he crossed the Rubicon river with his troops to start a civil war.

“It’s indicating that things have been put in motion and won’t be stopped,” Robinson said. It appears Pierson acted alone, but the sheriff’s investigation will determine whether anyone helped him.

Pierson, who turned 18 in September, bought the shotgun legally on Dec. 6 from a local sporting goods store and purchased the ammunition last week.

Police said he entered the school from the north side and fired randomly down a hallway before shooting Davis in the head at point-blank range. She was sitting in the hallway with a friend and had no time to react.

He then moved to the library, as a sheriff’s deputy acting as a school resource officer closed in. Murphy, knowing he was sought, escaped the library with the help of a school janitor. Robinson would not elaborate on his exit.

Pierson fired a total of five times and ignited a Molotov cocktail that set ablaze three bookshelves in the library. He then fatally shot himself in the head, about 80 seconds after he entered the school.

“I heard him say, ‘Where’s Murphy? Where’s Murphy?’ ” between gunshots, said senior August Clary, who was sitting in science class when the school went on lockdown.

Tait Priser, who graduated last year, said he was chatting with a teacher down the hall when he heard a gunshot and saw Pierson, whom he did not recognize, wearing a hooded sweatshirt and aiming the shotgun in their direction.

“I looked, saw him reload and dove into the atrium,” he said. “He wasn’t running, he wasn’t yelling. He was calmly walking. It just seemed like it was an everyday occurrence for him.”

Robinson said Pierson had been nursing a grudge against Murphy and had made a verbal threat against him in September that resulted in disciplinary action. Students have said Pierson and Murphy had ongoing confrontations about issues on the debate team.

Pierson’s father, Mark, told The Denver Post that he and his ex-wife attended multiple disciplinary hearings with school officials earlier this year.

The new information came as staff members prepared to return to the school to gather their belongings Wednesday. Students will be allowed to do the same Thursday. The district said classes and final exams won’t resume until after winter break in January.

Sadie Gurman: 303-954-1661, sgurman@denverpost.com or twitter.com/sgurman

Staff writer Jordan Steffen contributed to this report.