"She's obviously in a coma, in critical condition," Hickenlooper said of Davis, who is hospitalized at Littleton Adventist Hospital. "We all have to keep Claire in our thoughts and prayers. Her parents ... I can't imagine what they're going through. It's unspeakable."
Hickenlooper made his remarks on CBS' "Face the Nation."
About 500 classmates held a candlelight vigil Saturday for Davis, who was sitting with a friend near the school library when she was shot in the head. Arapahoe County Sheriff Grayson Robinson has said investigators think she was shot at random by Pierson, who had gone into the school looking for a teacher with whom he had a dispute.
Fellow students described Davis as a vibrant senior and equestrian with a lot of friends.
Pierson may have been nursing a grudge against the teacher -- a librarian and head of the school debate team -- since September. Pierson was on the team and had been disciplined by the librarian for reasons yet to be disclosed, the sheriff said. He said Pierson threatened that teacher in September and came to the school Friday intending to harm him and inflict numerous other casualties.
Pierson excelled at speech and debate and was passionate about the team, friends said. They described him as a smart student who apparently didn't shirk from confrontations in class.
"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.
Hickenlooper said that there are "strategies and protocols in place, where we had a deputy sheriff in the building who immediately ran towards the trouble."
"That's a remarkable response, and I think everybody from the sheriff out here, Grayson Robinson, his entire team, they deserve a lot of credit for what could have been much, much worse."
Arapahoe High officials also immediately instituted a lockdown -- something well-rehearsed at the school -- with teachers and students hiding in closets and locking classroom doors.
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 noted that Pierson was not a loner but cautioned that the investigation was in its early stages.
"He didn't seem to have a mental illness," the governor said. "He had a lot of friends, he was outspoken. But again, there's no rhyme or reason. We can't -- there's nothing that says, ah, now I understand."
Associated Press writer P. Solomon Banda contributed to this report.
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