ARAPAHOE COUNTY - New details are emerging about trouble at Arapahoe High School in the months leading up to the shooting.
A disagreement with a teacher led to disciplinary action against Karl Pierson.
Arapahoe County Sheriff Grayson Robinson says Pierson went to the school on Friday seeking revenge against that teacher, school librarian Tracy Murphy.
Few classmates knew the shooter like Joe Redmond, a friend and co-captain on the speech and debate team.
He says, looking back, there were warning signs.
Classmates say Pierson was punished in the past for using strong language and could be verbally combative.
"Whenever he would get angry, he would say 'Oh I just want to shoot everyone up," Redmond said.
Redmond and others assumed he was joking.
"Obviously he wasn't kidding," Redmond said. "Karl was a good guy. He made my days better. And that's what hurts the most is he wasn't a villain."
9NEWS has spoken to several students with different impressions of Pierson.
Some say he was friendly and outspoken.
Others say he made them nervous.
"You never think it's going to be your school," Redmond said.
Redmond, a senior, was hiding in a hallway during the shooting and listening to a school administrator's handheld radio.
"We heard a play by play of everything that happened," Redmond said. "And I heard it was Karl Pierson and I broke down because he's my friend."
Redmond says the shooter was one of the best people on the speech and debate team.
During a 2010 9NEWS town hall style debate, the then-freshman was selected from the audience to ask a question.
"What would you like your legacy to be," Pierson asked.
Concerned about his own legacy even then, he told friends he had big future plans that were centered around his passion for speech and debate.
His coach and mentor was school librarian Tracy Murphy.
"He went to nationals with Mr. Murphy and I know that they did not get along on this trip to nationals," Redmond said.
"He felt powerless," Redmond said. "Karl had threatened to kill Mr. Murphy, kind of half jokingly. Mr. Murphy brought that to the administration and Karl got suspended for that."
Redmond says his friend was never the same after that suspension in September.
"When Karl came back, he was pretty angry. He felt like the suspension had ruined his chances of getting into college and ruined his future," Redmond said.
Both teenagers also ran cross country together.
They had a race at Columbine last year, and even visited the shooting memorial together.
The former teammate's final conversation was about economics, just one day before the shooting.
The crime tape is gone from the shooter's home in Highlands Ranch. The home is located in a subdivision near Sugarstone Circle and Buttonhill Court.
A wooden board now covers the door that investigators, including ATF agents, kicked-in on Friday as they searched for evidence in the case.
(Copyright 2013 by The Associated Press. All Rights Reserved.)