CENTENNIAL, Colo. (REUTERS) - 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 he received months ago from the school's debate club coach, the county sheriff has said.
The pump-action shotgun and multiple rounds of ammunition used by the suspect, Karl Pierson, in Friday's shooting were legally purchased from local retailers, Arapahoe County Sheriff Grayson Robinson said on Saturday.
He said Pierson bought the shotgun on Dec 6 and the ammunition the morning of the shooting.
Pierson, a senior remembered by classmates as studious yet argumentative - and described by some as socially awkward - was 18 when he made the purchases, the minimum age for buying a shotgun or other types of rifles in Colorado.
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 point-blank range, although apparently at random, 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.
Davis and Pierson were not thought to have had any particular acquaintance with each other. "I believe she was simply in the wrong place at the wrong time," the sheriff said.
Robinson also said 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.
His body was found by law enforcement officers who were closing in on the youth but never fired a shot.
Investigators believe 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" for verbal threats other students reported that Pierson had directed 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.
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 librarian Tracy Murphy.
Students who knew Pierson said he was heavily involved in the speech and debate club, until he was placed on some kind of restriction by the coach.
"Speech and debate was his platform," senior classmate Dylan Johnson, 17, told Reuters outside the school on Saturday.
"When Mr Murphy took that away, I think that's what set him off, because he didn't have an outlet anymore."
The violence on Friday unfolded just 8 miles (13 km) from Columbine High School, where a pair of students shot 12 classmates and a teacher to death before killing themselves in 1999.
It also came on the eve of the anniversary of the Sandy Hook Elementary School massacre in Newtown, Connecticut, in which a gunman killed 20 first-graders and six adults before taking his own life.
So far this year, there have been 28 shootings on US school grounds during school hours, including Friday's incident at Arapahoe High, according to a tally kept by the gun control advocacy groups Mayors Against Illegal Guns and Moms Demand Action for Gun Sense.
Centennial is also where James Holmes, the former graduate student charged with shooting 12 moviegoers to death in nearby Aurora, Colorado, in July 2012, is being held in custody, awaiting trial on capital murder charges.
Following Friday's shooting, fellow students described Pierson as a smart and outspoken member of the school's track team and debate club, some recalling him as having a combative, even gawky personality.
"He was just this big, goofy kid," Ryan Curtin, 16, a sophomore who knew Pierson from a weight-training class they took together. "He was just kind of goofy and awkward, but never bullied that I could tell."
Zack Runberg, 18, another senior who took a world literature course with Pierson, said of him: "He was not a loner. He did have friends and would speak his mind in class."
Johnson said Pierson was known for speaking without "a filter," blurting out inappropriate comments that could get him in hot water. He said he was in Spanish class with Pierson when his classmate got angry and cursed at the teacher.
The Denver Post reported that Pierson placed third in an extemporaneous speaking contest in April, qualifying him for a spot in a national tournament in June.
After the June competition, according to the Post, he wrote on Facebook: "Hey guys! I just got back from day 2 of nationals and I'm sorry to say I am not moving on, nor am I in the top 60 in the country. Thank you from everyone for your support, and have a great rest of the summer and hope we can send some more guys to nationals in Kansas next year!"
The Post said court records showed Pierson's parents ended their marriage in a divorce finalised in August 2012.