Arapahoe County Sheriff Grayson Robinson said Karl Pierson likely was motived by retaliation against a faculty member - probably a librarian at the school - when he opened fire Friday at Arapahoe High School. Robinson said it appears the librarian was the initial target but that Pierson planned to hurt multiple people.
Robinson said at a news conference that the teen bought the pump-action shotgun legally Dec. 6 at a local store.
Anyone over 18 is allowed to buy a shotgun in Colorado; only those over 21 can legally buy a handgun.
Also Saturday, Robinson identified the wounded student as 17-year-old Claire Esther Davis. Robinson said Davis was seated with a friend near the door Pierson used to enter the school when Pierson shot her at point-blank range.
Davis wasn't a specific, intended target, the sheriff said.
"She is an innocent young lady, and she was an innocent victim of an evil act of violence," Robinson said. Davis remained hospitalized Saturday in critical condition.
Robinson said Pierson bought ammunition Friday and went to the school armed with multiple rounds. The sheriff believes the teen intended to harm many more people but ended up killing himself less than two minutes after entering the school because he knew a sheriff's deputy was closing in.