Student Storms Arapahoe High School With Shotgun Looking for Teacher

Colorado high school senior Karl Pierson targeted school librarian in recent school shooting.
3:00 | 12/14/13

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Transcript for Student Storms Arapahoe High School With Shotgun Looking for Teacher
We have more details on the school shooting in colorado on friday. Eerie flashback to columbine. We know more about his arsenal, the intended target. And the heroism in the midst of this crisis. Clayton sandell is in sentencentenn centennial, colora good morning. Reporter: This entire school behind me is a crime scene. Investigators working all night to find out what happened here when a gunman, a students gunman walked through the doors, making no effort to hide his shotgun. The terror began at 12:33 p.M. Shotgun shell on the ground. Assuming they have a shotgun. Reporter: When police say 18-year-old karl piersen, a senior, walked into his school for the last time. He had one target, tracy murphy, the school librarian. The kid went after him, specifically. He wanted to kill him. Reporter: Murphy runs the school debate team. Khtv reports that piersen was recently kicked off the team. This shooting was a result of revenge on the part of the shooter, because of a confrontation or a disagreement. Reporter: In the chaos, a 15-year-old girl was shot in the head. I have a student down in the athletic hall. She is bleeding. Reporter: She is in critical condition. But what murphy did next may have saved many more lives. Knowing he was the target -- I believe it was a wise, tactical decision for him to remove himself from the school. Reporter: About five minutes after it all began, as officers closed in, police say piersen shot himself. The suspect has been found inside the school. And he has deceased. Reporter: Overnights, murphy, the librarian, left his home, declining to talk. You doing okay? Reporter: At the same time, bomb squads and investigators began searching the home where police say piersen lived with his mother. This is the latest high-profile shooting for colorado, just 15 miles from last year's movie city massacre in aurora. And only eight miles from columbine high school. The headline in this morning's "denver post," again. Did you think this would happen at your kids' school? To be honest, there's not a day that they leave that I don't have a fleeting thought about that. Reporter: Police believe at this point that piersen, who was armed with that shotgun, along with a couple of molotov cocktails was acting alone. They do not believe they are connected to the newtown shooting. Just a coincidence. A coincidence. And that headline says it all. Again. Clayton, our thanks to you.

This transcript has been automatically generated and may not be 100% accurate.

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