HIGHLANDS RANCH, Colo. - Karl Halverson Pierson was the Arapahoe High School student gunman who shot a student inside the school while seeking "revenge" on a staff member, the Arapahoe County Sheriff confirmed Friday night.
Pierson, 18, went into the school armed with a shotgun looking for the school's librarian, Tracy Murphy -- who was head of the school's speech and debate team and who had recently kicked Pierson off the team, sources told the CALL7 Investigators.
"We were just in the library doing some work and we heard a loud bang. I thought it might have been someone dropping something, but then I heard two more," said senior Max Minne. "I knew it was a gun for sure."
Minne, who was studying in the library during his 5th period class, said he started to approach the front door of the library to see what was happening.
"The shooter just ripped through the door, started yelling, 'Where's Murphy?' -- That's the main librarian of the school. I saw him bust through the door of the library with a gun, so I just took off and started yelling, 'Everyone get out! Everyone get out!'"
Minne said he ran out of the library through a back door with other students. It led them through a classroom and into the hallway near the west entrance.
The hallway, Minne said, was filled with smoke.
"There was a girl on the ground and she was bleeding out of her stomach -- at least that's what I thought it was. I was sprinting so fast that I didn't really get a look at her. She was on her knees," he said. "I wish I would have done something. Tried to get her out. But everything was happening so fast, tunnel vision, just trying to get out of there as quick as I can."
The 17-year-old girl who was shot was airlifted to Littleton Hospital, where she is listed in critical condition, Arapahoe County Sheriff Grayson Robinson said.
Another student initially believed to have been shot was not injured -- she was just covered in blood from the first victim, Robinson said. She was transported to Swedish Hospital as a precaution on Friday, but has since been released.
Pierson was found dead from an apparent self-inflicted gunshot wound 5 minutes after the deputy assigned as a school resource officer made an initial radio call about the "active shooter" situation inside the school, Robinson said.
The faculty member -- Murphy -- escaped without injury.
7NEWS has learned that Pierson -- who was described as exceptionally bright but difficult and argumentative -- was upset about being booted from the debate team.
Minne said the senior had a combative personality.
"He was always the type that wanted to be right. He would always argue with the teacher about scholastic things, whatever we were talking about he always had something to say. Never wanted anyone to tell him he was wrong. I definitely noticed that in the classes I was in with him," Minne said. "He spoke intelligently, he's a smart guy, I think he did pretty well in school, he was just kinda weird. ... He wasn't a violent person, he was just verbally aggressive."
Pierson's neighbor said he was quiet and seemed like a nice boy but they didn't really know him well.
"It's really sad. It's unfortunate that he'd be so upset about being kicked off the debate team that he'd do something like this and it's unfortunate he'd have access to a weapon to be able to do something like this," said neighbor Randy Freitik.
Minee said Pierson had been sent to the administration office earlier this week for yelling at teacher because he thought he got locked out of the class.
Robinson would not confirm CALL7's report about the debate team but said "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."
Robinson said the 18-year-old had not been suspended from school.
"There was no warning, no threat beforehand," Robinson said.
Detectives are executing a search warrant at Pierson's Highlands Ranch home which he shares with his mother, looking for additional evidence.
They are also looking at his car and another home that he had temporary access to, Grayson said.
Minne said he's thankful to have made it out alive, but will never forget the terror he saw.
"The lockdown drills that we do -- no one ever thinks you're going to need them, but we did," Minne said."At Arapahoe High School, I would never have expected any such thing ... You don't even see threats around school, really of anything. It's not just anything that happens in Centennial."
"The way he busted into the library yelling, 'Where's Murphy?' seemed that he was only there for one person, but the other two kids he got hit, that's what makes no sense to me ... Why did he shoot the girl? That's my No. 1 (question.) Students get mad at teachers all of the time, I just don't get why he would shoot a girl just doing her own thing in the hallway. Makes no sense to me," Minne said.