Keith Coffman Reuters
4:19 p.m. CST, December 14, 2013

CENTENNIAL, Colo. (Reuters) - Police sought on Saturday to learn why a teenager remembered by classmates as a studious yet argumentative youth stormed through his suburban Denver high school with a shotgun and Molotov cocktails, opening fire on a fellow student before taking his own life.

Arapahoe County Sheriff Grayson Robinson said detectives were investigating revenge as a possible motive for Friday's shooting, the latest of more than two dozen outbursts of gun violence to shake U.S. school campuses this year.

But he has declined to elaborate, or to comment specifically on reports that the suspect, identified as 18-year-old Karl Pierson, was angry at having recently been demoted from or kicked off the school's speech and debate team.

Robinson said the student gunman's intended target appeared to be a teacher Pierson was calling out for by name as the armed teenager stalked through the hallways of Arapahoe High School.

The faculty member, whom police have not publicly named but who local media accounts, citing witnesses, have identified as school librarian and debate club coach Tracy Murphy, fled the building unharmed.

Students who knew Pierson said he was heavily involved in the debate club, until he was suspended from the team.

"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."

When the suspect started shooting, a 15-year-old girl, who authorities say was in Pierson's line of fire although apparently not an intended victim, was severely wounded.

A second girl who was initially thought to have been wounded was not injured, but spattered with blood from the teenager who was hit as staff and students scurried for cover.

Sheriff's deputies and police converging on the school encountered heavy smoke in the building from a Molotov cocktail-like incendiary device Pierson had set off, one of two he was carrying.

Pierson, who had entered the school brandishing a shotgun, was found by police about 15 minutes later in a classroom, dead from a self-inflicted gunshot wound, the sheriff said.

Robinson said the teenager appeared to have killed himself as law enforcement officers closed in, although officers never fired a shot.

LATEST OUTBREAK OF SCHOOL GUN VIOLENCE

The wounded girl, who has not been publicly identified, remained listed in critical condition at nearby Littleton Adventist Hospital, where she had surgery, a hospital spokeswoman said.

Authorities have not disclosed the nature of her injuries, but an account in the Denver Post cited radio transmissions from the first deputy on the scene, who reported finding a student with a "bad head injury" moments after the shooting began.

The violence on Friday unfolded just 8 miles from Columbine High School, where a pair of students shot 13 classmates and staff 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 U.S. 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.

Following Friday's shooting, fellow students described Pierson as a smart and outspoken member of the school's track team and debate club. Police said they knew of no prior threats or discipline problems with Pierson, although some students recalled him as having a combative personality.

"He was not a loner. He did have friends and would speak his mind in class," said Zack Runberg, 18, another senior who took a world literature class with Pierson.

Johnson said Pierson was known for speaking without "a filter" at times, blurting out inappropriate comments that could get him in trouble. He said he was in Spanish class with Pierson when his classmate got angry and cursed at the teacher.

Classmate Max Minne told Denver's ABC News affiliate that Pierson was "verbally aggressive" and had been sent to the administration office earlier in the week for yelling at a teacher because he thought he had been locked out of a class.

Authorities said they planned to search the suspected gunman's vehicle, which was left parked at the school, and two homes owned by his parents.

The Denver Post reported that Pierson placed third in a 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 finalized in August 2012.

(Reporting by Keith Coffman; Additional reporting and writing by Steve Gorman; Editing by Barbara Goldberg, Vicki Allen and Peter Cooney) 

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Police in Colorado probe motives of dead high school gunman