Colorado School Shooting

Parents stand outside Arapahoe High School in Centennial, Colo., where a student shot and wounded another before killing himself. (John Leyba / Associated Press / December 13, 2013)

CENTENNIAL, Colo. – Carrying a shotgun, a machete, a bandolier of ammunition and a backpack with three incendiary devices, Karl Halverson Pierson entered Arapahoe High School and launched 80 seconds of terror as he hunted his debate coach, who was also the school librarian.

New details emerged Saturday as authorities described the country's latest school shooting.

As soon as he entered the building Friday, Pierson fired a round down the hallway and another from point-blank range that critically injured one student. He fired a third round down the hall and entered the library, where he fired  again and set off one of the Molotov cocktails, igniting bookshelves.

As fire and smoke poured through the room, Pierson fired a fifth round and went into the back corner of the library, where he fired his last shot, killing himself.

“It is our strong belief that he came to that school with that weapon and with multiple rounds and his intention was to utilize those multiple rounds to cause harm to a large number of individuals,” Arapahoe County Sheriff Grayson Robinson told reporters Saturday. Pierson had legally purchased the weapon a week earlier, Dec. 6.

Robinson identified the injured girl as Claire Esther Davis, 17, a senior. Earlier reports described the victim as a 15-year-old freshman. There was no real relationship between Davis and Pierson, said the sheriff, who read a statement from her parents asking for privacy.

“Our beautiful daughter Claire Davis has severe head trauma as the result of a gunshot wound," the statement said. "She needs your continued prayers."

On Saturday, investigators and students struggled to understand what could have motivated Pierson, a hard-driving debater and athlete. Classmates said Friday that he had been bounced from the debate team after altercations with the coach.  But Robinson said Pierson was still on the team, though he and the coach had quarreled in September.

Robinson said Pierson had been disciplined but wouldn't give details. Pierson had threatened the coach at least once, Robinson said.

Natalie Black, a 17-year-old senior, said she was dumbfounded that the apparent target of the rampage was the school's debate coach, who has yet to be officially named.

“He's the nicest teacher I've ever known. He really looks out for kids,” Black said. “The only time I've ever heard him get mad was when you brought food or drink in the library.”

August Clary, also 17 and a senior, was equally puzzled. “Why on Earth him?”

The only person who could definitively answer that question was Pierson, variously described as strong-willed and with sharp political ideas and a desire to express them, characteristics that served him well during his debates.

On Saturday, Clary and Black were across the street from their school, which was still wrapped in yellow crime scene tape.

Clary said he has known Pierson since third grade. Pierson moved away, but in middle school they reconnected on Facebook and then began attending Arapahoe High School together as freshmen. Both were Eagle Scouts, and Pierson last year signed Clary’s yearbook: “Feels good doesn’t it?” referring to their achievement in climbing through the Scouting hierarchy.

But Clary said Pierson was often argumentative with teachers and classmates, seeming to enjoy the heat of disagreement, especially about politics. “He was an angry, strange kid, but he was very smart,” Clary said, adding that he was “super, super surprised” to learn the shooter was Pierson.

Clary said he was in fifth-period chemistry Friday when he heard an explosion about 50 feet away in the hallway. When more shots came, he knew it was gunfire. By then the fire alarm had been tripped and he could hear people screaming. Then there was one lone blast, which he now assumes was the self-inflicted shot that ended his friend’s life.

Clary and Black said Pierson posted a YouTube song link to Facebook on Wednesday with the caption: “It’s a good day for a suicide.”

Joe Redmond, an 18-year-old senior who was good friends with Pierson and was also on the debate team, praised his former teammate's debating prowess, saying Pierson was the best on the team.

“He and I talked politics and economics a lot. He was very good when he was on the team, and he knew what he was talking about,” Redmond said.

Pierson, he said, was a self-proclaimed socialist. “But he also wore a Communist Party T-shirt to confuse people,” Redmond said. Pierson also sometimes wore an Air Force Academy hoodie and apparently wanted to attend the school, Redmond said. His political leanings, friends say, were more antiauthoritarian than communist.

In addition to the debate team, Pierson ran cross-country for the school last year and the year before.

In a photo posted in November 2011 by the Littleton Public Schools Foundation, a lanky Pierson, wearing a black headband, shorts and a T-shirt with his runners’ number tacked on, poses with three other students who had finished running a charity 10K. Pierson, one arm around the shoulder of the student next to him, smiles and gives the camera a thumbs up.

Exactly why Pierson was angry at the debate coach remained unclear. But friends noted Pierson's altered behavior after the two had a falling-out. Pierson was angrier and cruder, making frequent racist jokes and acting with what Larson Ross, a debate team captain, described as “accusatory arrogance.”

“It seemed like he was attacking people to try and elicit a response, and in doing so he would put himself above that person on a mental level,” Ross, 18, said. “It started making it tough for a lot of people to be his friend.”

Changes in Pierson were so apparent that on the day of the shooting, Ross said his friends were talking about how the debater might be “going off the edge” and that they needed to “tell someone.”

But Ross’s friends did not have time to tell anyone. The attack happened during 5th period, just one hour after their discussion.

Ross said he remembers Pierson as a bright, goofy, hardworking guy who put his “heart and soul” into what he was doing -- whether it was running, debating or participating in his Advanced Placement courses.

Ross recalled that last year, when Pierson won a debate tournament, he did a  “Gangnam Style” dance on stage as he received the award.

He had at least one argument with the debate coach, but that was not the only time he had questioned authority, said Ross, describing one automated, multiple-choice test.

“Last year in health class, Karl wrote on Scantron, ‘fight the power,’ because he thought it was funny,” Ross said. “But the teacher took a really personal offense with it and took his grade down to a B and Karl got really upset and had a yelling match with that teacher.”

Four miles due south of Arapahoe High School, in a tidy subdivision of two-story, middle-class homes, Pierson’s house sat empty Saturday morning. There were no cars in the driveway, the newspaper was unfetched, and a piece of plywood replaced the front door that authorities broke down Friday afternoon when they went to search the home.

Jo Vandewege has lived across the street from Karl Pierson since 2008. She remembered Pierson as  “an American kid,” calling him funny, smart and athletic. He would display signs of support for Arapahoe High in the front yard and used to do chin-ups on the tree in the front yard. She did not know why he attended Arapahoe High rather than the school zoned for the area.

“It’s such a disquieting feeling to know that we could be so disconnected to people we know. He’s right there. We’ve all interacted with him. He must have been in some deep mental anguish and we didn’t know it, and no one reached out to him,” Vandewege, a 29-year-old graduate student, said.

She described the family as pleasant but mostly quiet. Pierson’s mother, with whom authorities say he lived, would welcome new arrivals to the Highlands Park neighborhood with a plate of cookies. Vandewege only heard through the grapevine the parents had divorced.

She was watching television coverage of the shooting Friday afternoon when agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives pounded on her door and warned her to stay inside. Soon afterward she heard someone shout: “This is ATF. Open up!” at the Pierson home.

When there was no answer, she said, she heard the door being broken down.

“That’s when my brain went: ‘Oh, Karl goes to Arapahoe High School.’ And then I knew.”

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saba.hamedy@latimes.com

soumya.karlamangla@latimes.com