CENTENNIAL, Colo. (CBS4) - Karl Pierson, an 18-year-old student killed himself inside Arapahoe High School after shooting and injuring another student early Friday afternoon.
Pierson shot and gravely wounded 17-year-old student Claire Esther Davis after storming into the school at 12:33 p.m. on Friday afternoon. Davis underwent surgery and was in critical condition Friday evening.
As of Saturday evening Davis is still in critical condition at Littleton Adventist Hospital with severe trauma to the head from a gunshot.
Arapahoe County Sheriff Grayson Robinson said Pierson was originally targeting a teacher. He said Pierson, who acted alone, entered the school with a shotgun and identified the teacher he was targeting.
"The gunman came into the school and immediately asked for the location of a very specific teacher, and he named that teacher by name," Robinson said.
"When the teacher heard that this individual was asking for him the teacher exited the school immediately, which was in my opinion the most important tactical decision that could have been made. He knew he was the target and he left that school in an effort to try to encourage the shooter to also leave the school with the focus of safety and security and well being of our students in his mind."
Robinson says the motive for the shooting appears to be retaliation. He said Pierson's debate coached disciplined him in September and Pierson verbally threatened him at that time.
"The threat was the result of a disciplinary action related to the debate team. The shooter was not removed from the debate team. The teacher that was responsible for that disciplinary action was the shooter's main target." said Robinson.
The sheriff said that Pierson was armed with a shotgun, a machete and had multiple rounds of ammunition strapped to his body and had three molotov cocktails in his backpack.
Investigators said Pierson started shooting as soon as he entered the school through an entrance adjacent to the library.
PHOTO GALLERY: Shooting At Arapahoe High School
He fired five rounds and detonated a molotov cocktail in the library setting fire to bookcases before taking his own life after law enforcement officers entered the school.
"I believe their quick response and their quick action saved lives in this incident. I have no way of knowing -- nor will I have a way of knowing -- but I believe the shooter knew that deputy sheriffs were immediately about to engage him. And I believe that that shooter took his life because he knew that he had been found."
According to the sheriff the entire shooting event was over in less than two minutes.
Some students that were inside the school when the shots rang out told CBS4 they hid in their classrooms.
Authorities say Pierson purchased the gun at a local retail outlet on Dec. 6 and purchased the ammo only a few days ago.
"They were safer inside their locked schoolroom classes than they would have been had we allowed them to exit, and that was part of our strategy and part of our protocol," Robinson said.
After Pierson was found dead students and faculty evacuated Arapahoe High School in groups, escorted by police. Some of the students experienced anxiety attacks during the evacuation.
Students were then taken by bus to Shepherd of the Hills Church at 7691 S. University Blvd. to reunite with family members.
The school is located at 2201 East Dry Creek near University. It is part of Littleton Public Schools and has more than 2,000 students.
The school will remain closed for several days as police investigate the scene of the crime.
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