A gunman has killed himself after opening fire at a Denver high school near to the site of the Columbine High School massacre.
One 15-year-old student was injured at the Arapahoe High School in Centennial and is in a critical condition, according to authorities.
Arapahoe County Sheriff Grayson Robinson identified the shooter as 18-year-old Karl Halverson Pierson.
Pierson entered the school with a shotgun and was looking for an individual teacher who he identified by name, said Mr Robinson.
Armed police at the school He added: "The teacher began to understand that he was being looked at and exited the school. One student confronted the armed student and was shot."
Mr Robinson did not elaborate on any possible motive except to say Pierson had had a "confrontation or disagreement" with the teacher.
The teenager was later found with fatal self-inflicted gunshot wounds. Mr Robinson said a possible Molotov cocktail was also found at the scene.
Pupils were led out of the building with their hands in the air by police officers after the shooting, which began at 12.30pm local time. All the schools in the area are on lockdown as a result.
One student told the Denver Post: "I was scared and shaking." She added that she heard, "bang, bang, bang" and by the third shot was on the ground.
Youngsters told the newspaper they hid in the corners of dark classrooms until police SWAT teams arrived.
A student is reunited with her father after the shooting The school is about eight miles (13km) east of Columbine High School in Littleton, where two teenage shooters killed 12 classmates and a teacher before killing themselves in 1999.
Tracy Monroe, who had step-siblings who attended Columbine, was standing outside the high school looking at her phone, reading text messages from her 15-year-old daughter inside.
She said she got the first text from her daughter, Jade Stanton, at 12.41pm. The text read: "There's sirens. It's real. I love you."
A few minutes later, Jade texted "shots were fired in our school".
Ms Monroe rushed to the school and was relieved when Jade texted that a police officer entered her classroom and that she was safe.
Ms Monroe was friends with a teacher killed in the Columbine shooting, Dave Carpenter.
"We didn't think it could happen in Colorado then, either," she said.
Some 2,141 students attend the school, which has 70 classrooms.
The attack comes almost one year after 20 children and six adults were killed in a shooting at Sandy Hook Elementary in Newtown.
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