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Karl Halverson Pierson walked into Arapahoe High School in Centennial, Colorado, on the afternoon of December 13, 2013, carrying a pump-action shotgun, a machete and a backpack of Molotov cocktails, and he was looking for one person. Investigators said the 18-year-old senior wanted to kill the school's librarian and debate coach, who had recently disciplined him. Pierson never found the teacher.

The attack lasted about 80 seconds. According to the Arapahoe County Sheriff's Office, Pierson came through a door near the library just before 12:33 p.m., fired a shot down a hallway, then shot 17-year-old senior Claire Esther Davis in the head at close range as she sat with a friend. Authorities said Davis had no part in his grievance and was not a target. He fired several more rounds, ignited one of the incendiary devices inside the library and then killed himself in a back corner of the room.

Students and staff members called 911 as the shooting began, and the recordings later released by authorities capture callers reporting gunfire inside the building and pleading for police. A deputy sheriff assigned to the school as a resource officer, James Englert, ran toward the shots with an unarmed security guard and two administrators. The sheriff credited that response with preventing further bloodshed, saying Pierson shot himself as the deputy closed in.

Davis never regained consciousness. She died on December 21, 2013, eight days after the attack, bringing the death toll to two. She and Pierson were the only people killed, and no one else was struck by gunfire.

Investigators said Pierson had planned the attack for months. He bought the shotgun legally about a week earlier and had gone bowling shortly before, according to the sheriff's office. In a September diary entry he wrote that he intended to "shoot up" the school by the end of the year, and on the day of the attack he marked his arm with the Latin phrase "Alea iacta est," meaning "the die is cast," along with numbers identifying five areas of the school he planned to target.

The shooting led to years of scrutiny of how the district handled warning signs. A report produced through an arbitration agreement between Davis's family and Littleton Public Schools, released in January 2016 by researchers at the University of Colorado Boulder and the University of Northern Colorado, found major failures in the district's threat assessment and information sharing in the months before the attack. Colorado lawmakers passed the Claire Davis School Safety Act in 2015, naming it for her and allowing victims to sue districts that fail to take reasonable care to protect students from violence.