Fellow students and neighbors say Karl Pierson's actions at the school were totally at odds with his usual behavior, CNN reported Saturday.
Officials said Pierson, a student at Arapahoe High School in the Denver suburb of Centennial, armed himself with a shotgun and went looking for one of his teachers.
The teacher was escorted out of school before Pierson located him. Pierson then shot and critically wounded a 15-years female student before turning the gun on himself Friday, officials said.
The female student was in critical condition late Friday following surgery.
Arapahoe County Sheriff Grayson Robinson said his deputies worked through the night to determine the reason behind the shooting, CNN reported.
"That is their key responsibility -- to find out about him, find out what his background was," he said.
A team of investigators planned to search Pierson's parents' homes Saturday.
Neighbors described him as a "nice young man," an athlete who ran cross-country and who had worked on an Eagle Scout project.
"He was the last person I would expect to shoot up a high school," said high school senior Frank Woronoff, who said he had known Pierson since they were freshmen. "He was honestly incredibly humble and down to earth. He was a little geeky but in a charming way."
Other students described Pierson as bright and whose family attended Bible study meetings, the Denver Post reported.
Pierson had also won first and second place on the school speech and debate team.
When Pierson entered the school with the shotgun, Robinson said, he was looking for his debate coach.
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