Transcript for High School Horror in Colorado: The Motive
Thanks to you. Now to the fast-moving investigation in colorado. Revealing the motive. What drove a student to target a librarian. We watched the all too familiar scene playing out, students with their hands in the air. A senior opened fire in the hallways. Tonight we're learning more about what he was armed with and the classmate fighting to survive. Clayton sandell in colorado. Reporter: Tonight 17-year-old claire davis is fighting for her life. Davis shot yesterday in the head. Police say fellow student 18-year-old karl pierson pulled the trigger. It was terrible. It's awful. And I'm sad. Reporter: Police say davis was not pierson's intended target. They believe he'd come to kill teacher tracy murphy. She runs the debate team. He had been in trouble with the team, allegedly making a threat in september. This shooting was the result of revenge on the part of the shooter because of a confrontation or disagreement between the shooter and the teacher. Reporter: Investigators said today that he legally purchased a 12-gauge shotgun at a denver area store within one week before the attack. The shooter purchased the shotgun on december 6th at a local retail outlet. The weapon was purchased legally. Reporter: The images friday of terrified students, so similar to the ones just down the road at columbine. This sergeant teaches that unlike columbine, first responders can't afford to wait to confront a gunman. Go immediately in, inside to address the threat. Reporter: That strategy, police say, saved lives here. Their quick response and their reaction saved lives. The second shot was aimed in our direction. And at that point, I was diving back into the classroom and pushing my friend and teacher in with me. Reporter: We learned he had extra am you nice and armed with a machete. From the moment he walked into this school, opened and fire and took his life was a minute and 20 seconds. The sheriff credits his troops for closing in fast, bringing the situation to an end before it got much worse.
This transcript has been automatically generated and may not be 100% accurate.