KUSA - The alleged Arapahoe High School shooter legally purchased the shotgun and ammunition he used in Friday's shooting.
During a press conference on Saturday Arapahoe County Sheriff Grayson Robinson said Pierson legally purchased the shotgun from a retail store on Dec. 6. On the morning of the shooting, Friday, Dec. 13, he legally bought ammunition.
In Colorado a person must be at least 18 years old to legally purchase a shotgun. Colorado law requires a person to be 21 years old to legally purchase a handgun.
According to Robinson, it all took less than 1 minute and 20 seconds from the time Pierson entered the school and fired the first round to the time he killed himself.
The 17-year-old girl shot by alleged gunman Karl Halverson Pierson has been identified as Claire Esther Davis.
Davis, a senior at Arapahoe High School, was allegedly shot by Pierson at point blank range
As of Saturday she is listed in critical condition at Littleton Adventist Hospital. She suffered severe head trauma.
Robinson said Davis is an innocent victim who was in the wrong place at the wrong time.
"I choose to never use his name again in public," Robinson said of the shooter.
During Saturday's press converence, Robinson said Pierson entered the high school just before 12:33 p.m. on Friday with a shotgun, multiple rounds of ammunition, a machete and a backpack with 3 incendiary devices.
Robinson said Pierson entered the north side of the school with his weapon clearly visible around 12:33 p.m. Friday and asked for Tracy Murphy by name. Robinson says Pierson had threatened that Murphy, the school's debate coach and librarian, back in September.
"The threat was the result of what I believe to be to an appropriate disciplinary action related to the debate team. Again the shooter was not removed from the debate team and the teacher who was responsible for that disciplinary action was his - the shooter's main target," Robinson said.
Robinson made it clear that Pierson was not suspended or kicked off the debate team because of the threat he made.
ACTIVE SHOOTER PROTOCOLS
Staff member, Tracy Murphy, after learning Pierson was looking for him and was armed, immediately implemented "active-shooter protocols."
Active shooter protocols include evacuating the grounds in a safe manner and placing the school on lockdown.
SEARCH FOR SHOOTER
About 20 minutes after police began searching Arapahoe High School, police found Pierson dead in a classroom from an apparent self-inflicted gunshot wound. His body was recovered from the school early Saturday morning.
Investigators did not engage in a gunfight with Pierson. They believe he acted alone.
Arapahoe County Sheriff investigators are looking into revenge against Murphy as a possible motive.
Friday night, an Arapahoe County deputy escorted Murphy home to pick up a few belongings.
DEVICES FOUND IN SCHOOL
Sheriff Robinson says Pierson brought 3 Molotov cocktails into the school in his backpack. He detonated one of them in the library.
"When the individual entered the library he fired one additional shot and then detonated or ignited one of the incendiary devices which caused a fire - a large amount of smoke inside the library and then caused at least three bookshelves to catch fire which generated additional smoke," Robinson said.
The other explosive devices were not detonated. A Molotov cocktail is a bottle-based improvised incendiary weapon.
Some students said Pierson yelled, "I'm not perfect," after Davis was shot.
On Friday students were seen walking toward the school's running track with their hands in the air on Friday. SKY9 showed students being patted down by police.
"The kids have been through a traumatic situation," Arapahoe County Sheriff Grayson Robinson said on Friday. "We need to ensure that anyone we evacuate isn't part of our problem. We're doing the evacuation slowly and deliberately."
Some friends say Pierson spoke of a desire to 'shoot everyone up.'
Pierson was described by other students as an intelligent, normal, and likeable guy.
Neighbors said he was a "nice young man."
Pierson was a routine winner in speech and debate contests. Students described him as a talented debater.
He was often seen posing with first and second place trophies. He also ran cross country. Two years ago, he was working on an Eagle Scout project.
LESSONS LEARNED AFTER LIVES LOST
The rapid response by officers is a direct result of lessons learned at Columbine in 1999, Platte Canyon in 2006 and the Youth With a Mission shooting in 2007.
Arvada Police Sergeant AJ DeAndrea was on the ground for all three.
The attack reopened old wounds in a state that still remembers the mass shootings at Columbine High School in 1999 and the Aurora movie theater shooting in July 2012. It also happened one day before the one year anniversay of Newtown, Conn. elementary school massacre.
VIGIL FOR DAVIS
A candlelight vigil was held for Davis on Saturday night. Hundreds of students, parents and community members filled Arapahoe Park to support each other and pray for Davis and her family.
ARAPAHOE HIGH SCHOOL
Arapahoe High School is a part of Littleton Public Schools. There are 70 different classrooms in the high school which covers a 254,000-square-foot area. There are 2,229 students that attend the high school. The school was built in 1964.
(KUSA-TV © 2013 Multimedia Holdings Corporation)