School shooting at Arapahoe High School in suburban Denver

Parker Semin, a 2011 Arapahoe High School graduate, prays at a makeshift memorial bearing the name of wounded student Claire Davis, who was shot by a classmate during school three days earlier in an attack, in front of Arapahoe High School in Centennial, Colo., Monday, Dec. 16, 2013. Semin said he came by to pray for the "speedy recovery" of Davis, age 17, who was shot in the head at close range with a shotgun, and remains in a coma. (AP Photo/Brennan Linsley)

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A teenager who may have had a grudge against a teacher opened fire Friday with a shotgun at a suburban Denver high school, wounding a fellow student before killing himself.

Quick-thinking students at Arapahoe High School alerted the targeted teacher, who quickly left the building. The scene unfolded on the eve of the Newtown massacre anniversary, a somber reminder of the ever-present potential for violence in the nation's schools.

The wounded student, a 15-year-old girl, underwent surgery and was in critical condition. Authorities originally said a second student was wounded, but Arapahoe County Sheriff Grayson Robinson said Friday night that the other girl taken to a hospital was covered in blood from the first student and wasn't injured.

Robinson identified the shooter as Karl Halverson Pierson, 18. The sheriff did not elaborate on any possible motive except to say Pierson had a "confrontation or disagreement" with the teacher. He didn't know if the injured girl was intentionally shot. (AP)

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