The teenage gunman who badly wounded a fellow student then shot himself to death at a suburban Denver high school apparently acted in retaliation for discipline levelled against him by the school's debate club coach, the county sheriff said on Saturday.
The pump-action shotgun and multiple rounds of ammunition used by the suspect, 18-year-old Karl Pierson, in Friday's shooting were legally purchased by him earlier this month from local retailers, Arapahoe County Sheriff Grayson Robinson said.
The sheriff's comments at an afternoon news conference shed new light on circumstances of a shooting that unfolded in less than a minute and a half and marked the latest of more than two dozen spasms of gun violence to shake US school campuses this year.
The faculty member believed by investigators to have been Pierson's intended target managed to flee the school unharmed. But the 17-year-old classmate gunned down at close range remained hospitalised in critical condition, the sheriff said.
The victim, a fellow senior identified as Claire Davis, whose age was mistakenly given by authorities on Friday as 15, was shot in the head, her family disclosed in a statement read at the news conference.
Robinson also said that Pierson stormed into the school carrying a machete, an ammunition belt strapped cross his chest and a backpack loaded with three Molotov cocktails, one of which he detonated in the school library moments before shooting himself in a corner of the room with his own shotgun.
Investigators believe that Pierson acted out of a grudge he held against the school librarian, who also serves as the faculty adviser for the school's speech and debate team to which Pierson belonged, the sheriff said.
According to Robinson, the librarian imposed some unspecified disciplinary action in September against Pierson "related to the debate team" in response to verbal threats other students reported Pierson directing at the faculty member.
But the sheriff said he did not believe the punishment was "overly harsh," and he denied Pierson had been suspended or ejected from the debate team, as a number of students have said in accounts to Reuters and other media outlets.
Robinson did not detail the nature of the verbal threat attributed to Pierson that got him in trouble.
The sheriff said Pierson repeatedly called out the faculty member by name as he stalked through the hallways of the school on Friday. The faculty member has not been publicly named by authorities, but local media accounts and students have identified him as Tracy Murphy.