we're learning new details about what played out in that high school. What happened in those 80 seconds of horror. This is hundreds of students and neighbors gathering at a vigil, in the shadow of the... See More
we're learning new details about what played out in that high school. What happened in those 80 seconds of horror. This is hundreds of students and neighbors gathering at a vigil, in the shadow of the school this weekend. Tonight, we hear from the young woman who went to the prom with the gunman. Were there any warning signs? Abc's clayton sandell with the interview. Reporter: Inside arapahoe high today, a testament to terror. Student backpacks, cell phones and computers lie scattered. The school is locked down. Reporter: Abandoned friday when police say 18-year-old karl pierson walks in a north entrance, opening fire with a 12-gauge shotgun. His second shot, aimed point blank at 17-year-old claire davis. She was an innocent victim of an evil act of violence. Reporter: Pierson moved down a hallway into the library, firing again. One paul 31, I'm going to the library. Reporter: The campus deputy sheriff and an unarmed security officer race toward the sound of gunfire as pierson lights a molotov cocktail he carries in a backpack. The shooter knew that the deputy was in the immediate area and while the deputy was containing the shooter, the shooter took his own life. Reporter: The attack lasts -- just 80 seconds. Investigators are still trying to sort out why pierson, an outspoken member of the debate team, apparently threatened then came to kill the team's coach, librarian tracy murphy. Bailey callahan was pierson's friend, his prom date junior year. Callahan, prom? I'm like, sure. Reporter: She talked to him thursday for the last time. He said, you know, you're one of the cool ones, callahan. Reporter: She saw no sign of what was about to come. He's not a monster. He's -- he's one of the best people I knew. He's not this -- this killer. He's -- he's karl pierson, one of the best people I know. Reporter: Tonight, davis clings to life. A senior with lots of friends, who loves to ride horses. Her friend chris davis is raising money for her. It's just weird to think that someone at our school, like a friend, a fellow student, just, that they're hurting and that it happened to them. Reporter: And tonight, claire davis is still in critical condition.
This transcript has been automatically generated and may not be 100% accurate.