Two people have been injured in a shooting at a high school near Denver, Colorado, US media report.
Police have said there is an "active shooter situation" inside Arapahoe High School in Centennial, Colorado.
The school and others in the area have been placed on lockdown.
The shooting was reported the day before the anniversary of the shooting in Newtown, Connecticut, in which 20 schoolchildren and six workers were killed at a primary school.
The terms "lockdown" and "shelter in place" were once only used during prison riots and in emergency manuals. Today they are commonly used in US schools.
The mass shooting at Sandy Hook Elementary School in Connecticut on 14 December 2012 sparked a new debate about how to keep children safe - and whether lockdowns are the best response to a threat at or nearby a school.
There is no official database on the number of lockdowns, making it difficult to track whether they are on the increase. The BBC found news reports of at least 130 separate incidents where schools or college campuses were "locked down" in the month between 9 November 9 and 9 December.
Are these lockdowns evidence that schools are overreacting (or acting out of "abundance of caution") or proof they are better prepared for the worst? David Botti reports.
“If you trust in yourself, and believe in your dreams, and follow your star. . . you'll still get beaten by people who spent their time working hard and learning things and weren't so lazy.”
The suspect entered Arapahoe High School in Centennial at 12:33 local time (19:33 GMT), according to officials.
The BBC's Jonny Dymond: "The school is just eight miles from Columbine"
The school and others in the area were placed on lockdown.
Upon the initial reports of a shooting, heavily armed police trained in "active shooter" scenarios stormed the building, ordering students outside with their hands over their heads.
The student who confronted and was shot by the suspect was in serious condition and undergoing surgery in hospital, the sheriff said.
Inside, they found another student who had apparently suffered a minor gunshot wound.
Authorities also found the student believed to be the gunman, dead from an apparent self-inflicted gunshot wound, Arapahoe County Sheriff Grayson Robinson said.
"That individual is currently deceased and he apparently killed himself," he added.
Police have not yet released the name of the suspect but believe that he acted alone.
"Currently deceased"? That situation may change?
“If you trust in yourself, and believe in your dreams, and follow your star. . . you'll still get beaten by people who spent their time working hard and learning things and weren't so lazy.”
