CENTENNIAL, Colo. — An 18-year-old student who wounded a fellow student before killing himself at a suburban Denver school entered the building with a shotgun, a machete and three incendiary devices in his backpack, and had ammunition strapped to his body, police said on Saturday.

Arapahoe County Sheriff Grayson Robinson said Karl Pierson was motived by retaliation against a faculty member – probably a librarian – when he opened fire Friday at Arapahoe High School.

The sheriff says it appears the school librarian was the initial target but that Pierson planned to hurt multiple people.

Robinson said the librarian is leader of the speech team and Pierson had been a member before being disciplined by the coach in September.

Robinson also said Pierson bought the shotgun legally Dec. 6 at a local store.

Investigators said they recovered two Molotov cocktails, one of which had detonated.

Police said Pierson entered the school with the shotgun about 12:30 p.m. and began roaming the halls, shouting the name of the person he was pursuing.

According to the Associated Press, the librarian left the school without confronting Pierson.

Contrary to earlier reports, there was only one gunshot victim, a 15-year-old girl, Robinson told reporters. “There’s no reason to believe she was a target,” the sheriff said, adding that it was unclear if the girl tried to confront him.

A second girl was mistakenly thought to have been shot, but she had blood on her only because she came into contact with the injured girl, Robinson said.

The wounded student remained in serious condition in a local hospital.

The rest of the high school was evacuated, with students filing out with arms raised – an image reminiscent of other school shootings.

The shooting in Centennial, a city of about 100,000 not far from Denver, reminded many of the deadly 1999 attack at nearby Columbine High School in which two students killed a dozen classmates and a teacher.

Friends described Pierson as an intelligent, likable youth who last year was an officer in the debate club, serving as one of its captains. He reportedly placed third in a regional competition that allowed him to compete at a national tournament.

Joe Redmond, an 18-year-old senior, was one of three co-captains on the debate team, and said Pierson was a good friend whom he had known for about three years.

“At the end of the day Karl was a good person, and I hate the idea that he might be defined by his ending acts,” Redmond said. “I loved to be around him. I loved to talk to him. It’s always a shame when you hear friends making bad choices, but this one was particularly devastating.”

Redmond said Pierson was suspended from school in September for a few weeks after threatening the coach.

“When he came back he was really, really angry,” Redmond said.

The debate team won first place in a tournament last Saturday, Redmond said, and when students returned to school on Monday, Redmond asked Pierson if he would think about rejoining the team.

Pierson told Redmond that he still hated the teacher, and said, “Apparently you get suspended for threatening to kill a teacher.” He said Pierson went cold at that point, looked at Redmond, and then looked at the ground.

Redmond said Pierson and the teacher had argued over changes Pierson had wanted to make to the debate team.

“He doesn’t take a no very lightly,” Redmond said.

Redmond said his team’s victory on Saturday may have made Pierson upset that he was no longer on the team. He said Pierson had been the team’s best debater.

Redmond said Pierson also seemed to be affected by the divorce of his parents within the last two years.

“He was honestly a really good guy,” he said. “He and I talked politics and economics a lot. He was very good when he was on the team.”

After the shootings, “I broke down in tears,” Redmond said. “Friends firing guns in a school building, I never expected that to happen in my life.”

Pierson was active on social media, filing his most recent public Facebook update on Dec. 5, when he changed his cover photo to a melded image of the recently deceased actor Paul Walker, the late former South African leader Nelson Mandela, and Brian the dog from the television show “Family Guy.”

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