"He's a funny kid. He's smart. He's in the Eagle Scouts, a very intelligent kid. Did not like being wrong," said August Clary, who was a friend of Pierson. "If you're arguing with him, it's going to be, that's a feat if you win an argument against him."
"He would not be afraid to tell someone how he feels," said Zach Runberg, 18, a senior in Pierson's English class.
Pierson legally bought a shotgun on Dec. 6 at a local store, and he purchased ammunition the morning of the shootings. He managed to ignite a Molotov cocktail inside the school library before he killed himself as a fast-acting school security officer, a deputy sheriff, closed in, Robinson said.
That officer's aggressive response prevented more casualties, Robinson said. It's a tactic adopted nationwide after Columbine, in which first responders cordoned off the school before pursuing two student gunmen inside. The two killed 12 students and a teacher before killing themselves.
"It's nice to see how well the system worked. It's a remarkable improvement from before. This could have been much, much worse," Hickenlooper said.
After the Aurora, Colo., theater shootings and the Newtown, Conn., school shootings, Colorado's Democrat-led legislature this year implemented gun control measures that limited the size of ammunition magazines and instituted universal background checks. Colorado also appropriated more than $20 million for mental health hotlines and local crisis centers.
The measures were intended to address violence associated with so-called assault rifles, not shotguns that are widely owned for hunting and sport.
Hickenlooper acknowledged the latest shooting raised again questions about guns and violence. But he noted that Pierson "didn't seem to exhibit any signs of mental illness," and he cautioned that the investigation was in its early stages.
"Everyone in Colorado is asking the same questions," the governor said. "On the one hand there is a deeply held conviction for the freedoms laid out in the Second Amendment, but also a very, very strong conviction about the safety of children and the safety of the community."
Friday's shooting, he said, "defies any explanation, and you know we are searching for some pattern."