Police were looking into the motive of a student who opened fire at a Colorado High School, severely wounding a teenager before apparently killing himself, as the US state sought to understand the latest in a series of shootings.
Karl Pierson, 18, entered Arapahoe High School in a Denver suburb around midday on Friday brandishing a shot gun and asked fellow students about the location of a teacher. He then shot a 15-year-old girl who was nearby, a county official said.
He was later found inside a classroom with an apparently self-inflicted gunshot wound. The teacher, who quickly fled the school, was unharmed.
Fellow students said Pierson was a smart and likeable member of the school's track team and debate club.
Arapahoe County Sheriff Grayson Robinson said detectives were investigating "revenge" as a possible motive, but did not elaborate, though investigators believed the youth acted alone.
Police said they knew of no prior discipline problems.
The shooting in the Denver suburb of Centennial took place just 8 miles (13 km) from the scene of one of the deadliest school massacres in US history, Columbine High School, where two students gunned down 13 classmates and staff before killing themselves in 1999.
There was no indication the incident was related to the anniversary on Saturday of last year's Sandy Hook Elementary School shooting in Newtown, Connecticut, in which a gunman massacred 20 children and six adults before killing himself, Robinson said.
"I believe the shooter knew that deputy sheriffs were immediately about to engage him, and I believe that shooter took his life because he knew that he had been found," the sheriff told a news conference on Friday.
He said a firebomb-like device was detonated inside the school by the suspect, but a second such incendiary device did not go off and was rendered harmless by authorities.
Nearby businesses were evacuated as dozens of police arrived at the scene with guns drawn. They never fired their weapons as they pursued the gunman and evacuated the school.
Reports and images from the school show frenzy and fear among the students. Some could be seen being funneled out with their hands raised onto a track field where they were being patted down by police.
Holly Schaefer, an 18-year-old senior, said she saw blood on the hallway floor as students were being escorted out of the building.
Whitney Riley, 15, told CNN she and several other students and teachers hid in a utility room after hearing gunfire and did not come out until police arrived.
"We were shaking. We were crying. We were freaking out. I had a girl biting my arm," she said. "We stayed quiet and we heard a whole bunch of sounds. We heard people yelling. We heard walkie-talkies."
Colorado Governor John Hickenlooper, who pushed through tougher firearms legislation this year following the Newtown shooting and last year's attack in a Colorado movie theater that killed 12 people, called the shooting an "all-too-familiar sequence, where you have gunshots and parents racing to the school and unspeakable horror in a place of learning."
Authorities said they planned to conduct searches of the suspected gunman's vehicle, which was left parked at the school, and two homes owned by his parents.
The local ABC News affiliate in Denver reported the suspected gunman was upset after being kicked off the debate squad.
Arapahoe senior Frank Woronoff told CNN the gunman had recently been "demoted" on the debate team and the teacher he was said to be targeting was its faculty adviser and the school's librarian.