A student who shot two others before apparently killing himself at a Colorado high school is believed to have acted alone.
Arapahoe County Sheriff Grayson Robinson said the gunman entered Arapahoe High School in Centennial, near Denver, today armed with a shotgun and looking for a teacher he identified by name.
The teacher immediately left the school when he learned the student was looking for him but the gunman then shot two students, leaving one in a serious condition at a hospital. The other is also in hospital with a minor gunshot wound.
"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 said.
The seriously injured student was in surgery at a nearby hospital, said Jason Dunkel, spokesman for Littleton Adventist Hospital.
Police were outside the school, and students were seen walking toward the school's running track with their hands in the air. Television footage showed students being patted down.
The school is about eight miles east of Columbine High School in Littleton, where two teenage gunmen killed 12 classmates and a teacher before killing themselves in 1999.
The shooting also came a day before the first anniversary of the school massacre in Newtown, where a gunman killed 20 children and six adults at Sandy Hook Elementary School in Conneticut.
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.
Mrs Monroe said she got the first text from her daughter, Jade Stanton, at 12:41 p.m. The text read, "there's sirens. It's real. I love you"
A few minutes later, Jade texted "shots were fired in our school."
Mrs Monroe rushed to the school and was relieved when Jade texted that a police officer entered her classroom and that she was safe.
Mrs Monroe was family friends with a teacher killed in the Columbine shooting, Dave Carpenter.
"We didn't think it could happen in Colorado then, either," Mrs Monroe said.
More than 2,100 students attend Arapahoe High School.