CENTENNIAL, Colo. -- The teenage student police say opened fire at his Colorado high school legally purchased a 12-gauge shotgun at a Denver-area gun store within one week before Friday's attack, according to multiple law enforcement sources.
Karl Halversen Pierson, 18, barged into Arapahoe High School in Centennial with a shotgun on Friday, set off an explosive device, shot one student in the head, then killed himself. He was seeking "revenge" on the school's debate team instructor, Tracy Murphy, according to police.
Pierson had asked students where to find Murphy, according to police.
"The kid went after him specifically," the school's custodian Fabian Llerenas said. "He wanted to kill him."
Llerenas said he saw Halversen enter the building carrying a shotgun slung over his shoulder.
"I did see a young man ... entering the building, running kind of military style and I looked and I double looked and [he was carrying a] shotgun," Llerenas told ABC News affiliate KMGH-TV in Denver.
Pierson would have been legally allowed to buy and own a shotgun after he turned 18 on Sept. 3, sources said. The sources asked not to be identified because they are not authorized to speak publicly about the case.
Murphy runs the school debate team, and Pierson was recently kicked off the team, KMGH reported. Overnight, Murphy left his home declining to speak to reporters.
"This shooting was the result of revenge on the part of the shooter because of a confrontation or a disagreement between the shooter and the teacher that the shooter was looking for when he entered the school just before 12:33," Arapahoe County Sheriff Grayson Robinson told KMGH.
A 17-year-old girl, who has not been named, was in critical condition at a local hospital after she was shot in the head by Pierson during the incident. The girl was originally incorrectly identified by Robinson as 15 years old.
Another student was taken to the hospital with what were thought to be wounds, but it was later determined she was not injured, and had blood stains from the other victim, police said. The name of the student who was injured was not released, and it was unclear whether Pierson intentionally targeted her.
Police initially said one of the students was wounded while confronting the gunman, but Robinson said it was no longer clear how the students got shot.
Three other people suffered from anxiety attacks: One was transported, and the others were treated and released, police said.
Robinson said it was just five minutes between the time the school resource officer reported the shooting and found the suspect's body. As officers closed in, Pierson turned the shotgun on himself and died at the scene, according to police.
One explosive device, described as a Molotov cocktail, was detonated inside the school, the sheriff said, while a second device was later disabled by authorities. At the same time, bomb squads and investigators began searching the home where police say Pierson lived with his mother.
"It was clear that he was armed with a shotgun, no effort to hide or conceal it," Robinson said.
Tait Priser, a witness, told ABC News the suspect was wearing a black face cover and a black hoodie.
As shots rang out, the school was immediately placed on lockdown, with teachers dimming the lights and locking their classroom doors.
"I was in math and it happened three classrooms away," student Berny Acosta said from a nearby church where evacuated students assembled.
"There was a first shot and it happened from a classroom, and then I heard three or four more. There was a shot and then a pause and then four continuous shots. We went immediately into lockdown. ... Turned all the lights off, got as far from the windows and doors as we could," Acosta told ABC News.
"I saw blood on the ground of the study center when evacuated. That's where there was a lot of blood on the ground. That's where the ambulance carried people out from," he said
The school is part of the Littleton school district and is just a few miles from Columbine High School, the scene of one of the country's worst school massacres, in which 15 people, including two teenage gunmen, were killed.
The shooting also took place on the eve of the first anniversary of the Sandy Hook school shooting in Newtown, Conn., in which 27 people -- most of them first graders -- were killed.
In a scene that was eerily reminiscent of the Columbine tragedy, hundreds of students could be seen fleeing the school and then approaching police with their hands up to be searched.
Investigators questioned the gunman's family and acquaintances but said they do not believe there are other suspects.