Students returned to classes at Arapahoe High School on Tuesday.
CENTENNIAL, Colo. - A sheriff said the teenager who opened fire Friday at Arapahoe High School, wounding a fellow student before killing himself, entered the school with a shotgun, machete and three incendiary devices in his backpack and wore a bandolier filled with ammunition strapped to his body.
Arapahoe County Sheriff Grayson Robinson said 18-year-old Karl Pierson bought the gun legally at a south Denver store Dec. 6.
Anyone over the age of 18 is allowed to buy a shotgun in Colorado. Only those over 21 can legally buy a handgun.
Robinson said the shooting was likely motived by retaliation against the school's librarian, Tracy Murphy, who was head of the school's speech and debate team.
The sheriff identified the wounded student as 17-year-old Claire Esther Davis.
Davis remains in critical condition at Littleton Adventist Hospital. She was rushed into surgery Friday after suffering from a gunshot wound to her head.
Robinson said that based on new evidence, investigators believe, "Pierson was intent on causing harm to a large number of individuals."
"Certainly his motivation to come here yesterday was the upset that he had with the faculty member and we believe the faculty member was his initial target, but we believe that based on the fact that he was armed with multiple rounds, armed with a machete and armed with three incendiary devices, his intent was evil and his evil intent was to harm multiple individuals," Robinson said.
-- Entire incident lasted 1:20 --
Investigators said the total time, from Pierson the entering school until he took own life, was 1 minute and 20 seconds.
According to Robinson, investigators looking at surveillance video have pieced together what happened in the 80 seconds it took for the gunman to inflict terror and end his own life.
Pierson parked his vehicle in the north parking lot on campus and entered Arapahoe High through the north doorway, adjacent to the library, at 12:33 p.m.
Pierson made no effort to conceal the pump-action shotgun he was carrying when he entered through the school's doorway, Robinson said.
Based on surveillance video, authorities have determined that after entering the school, Pierson fired one round into a hall.
Robinson said Pierson then shot Davis, a senior at the school, at point-blank range and continued down the hallway.
Pierson entered the library after firing another shot in the hallway, and then lit one of the Molotov cocktails in his backpack -- which started a fire in the library, Robinson said.
According to Robinson, Pierson fired another shot in the library -- the fifth round from the shotgun -- and ran to the back corner of the library and took his own life.
Robinson said Pierson shot himself once he knew law enforcement officers were near.
The sheriff said he believes more loss of life was prevented because teachers, staff and law enforcement officials followed the active shooter protocol.
He said the investigation into the shooting is ongoing and as there is still a great deal of forensic work still left to do inside school.
He confirmed that the disagreement between the librarian and the gunman occurred in September, and it was related to the speech and debate team, which the librarian headed.
Pierson was not suspended from the team but suffered a "disciplinary action," Robinson said.
At that time, Pierson verbally threatened the staff member, and that was reported to authorities but it's not clear how that threat was followed up, and what occurred over the past two months, Robinson said.
However, the sheriff said based on evidence gathered at the gunman's home, he had planned the attack alone. His mother was out of town during Friday's attack.
"We are confident that the shooter was the lone participant, we do not believe there were other co-conspirators. We do need to look further into his lifestyle to ensure that no one else knew about this specific plan. But, in terms of other actors yesterday or immediate co-conspirators, there are none," Robinson said.
A longtime neighbor of Pierson said he was very intelligent and polite and saw him grow up for more than 15 years in a very loving home.
"I would just say, even today, that they're really good people. I don't know what happened inside him that made him do that but they're good people. They deserve a lot of sympathy right now," a neighbor said.
However, classmates said Pierson was argumentative and always had to be right, even over petty things.
The sheriff also made it clear that he won't have any part in glamorizing what teen gunman did and, after Saturday, will no longer say his name.
"I am no longer inclined nor will I speak his name in public. He is someone who victimized an innocent young lady by an act of evil. And in my opinion, he deserves no notoriety and certainly no celebrity," Robinson said.
Robinson said the school building will be released back to the control of the school district by Sunday afternoon.
However, classes have been canceled for Monday.
Coverage: Arapahoe High School Shooting
Teachers and staff went back to work at Arapahoe High School on Monday. Students will return to classes on Tuesday.
An online petition drive is asking Littleton Public Schools to give Arapahoe High School shooting victim Claire Davis a diploma at graduation in May.
When an armed student went into Arapahoe High School on December 13, many people sprung into action.
Claire Davis' father asked people to forgive the fellow student who shot and killed his 17-year-old daughter at Arapahoe High School.
The Arapahoe High School shooter entered the school through a door that should have been closed and locked, but was left ajar, Arapahoe County Sheriff Grayson Robinson said.
Gov. John Hickenlooper orders flags lowered to half-staff Wednesday in honor of Claire Davis, the student slain in the Arapahoe High School shooting.
Tickets to the celebration of Claire Davis' life are now available for students and staff at Arapahoe High School.
Claire Davis will be celebrated next week with a memorial at the National Western Stock Show and Complex.
Prosecutors are warning people that scam artists may try to take advantage of the shooting death of Claire Davis at Arapahoe High School by setting up phony websites and requesting donations.