At least seven students were wounded on Tuesday in a shooting at a Denver school, police said.
They said two suspects were in custody and the hunt was on for a possible third.
Deputies from the Douglas County Sheriff’s Office responded to reports of gunfire at a science and technology school in Highlands Ranch, 40 kilometres south of Denver, shortly before 2pm local time.
Seven students, and possibly an eighth, were taken to a hospital with injuries, said Douglas County Undersheriff Holly Nicholson-Kluth. It was unclear whether there were any fatalities, Ms Nicholson-Kluth said.
A man who called local radio station KOA said that two people burst into his son’s classroom and opened fire, wounding two pupils.
The father said the victims were alive and that pupils disarmed the shooters.
A pupil told Denver station KUSA-TV outside the school after the shooting that the violence had left him “a bit shaky” and scared, but “glad that I didn’t get hurt".
Colorado Governor Jared Polis said he was sending more state law enforcement officers to the scene.
“We are making all of our public safety resources available to assist the Douglas County Sheriff’s Department in their effort to secure the site and evacuate the students,” Mr Polis tweeted.
The violence in Colorado came a week after a gunman, aged 22, opened fire on the Charlotte campus of the University of North Carolina, killing two people and wounding four before he was disarmed and arrested.
The Colorado school, which combines elementary, middle and high school on one campus, has 1,850 students, the district said.
There was no immediate information of the ages or identities of the victims or suspects.
The sheriff’s department directed parents to a nearby recreation centre to pick up their children.
Dozens of police and fire vehicles surrounded the school as officers conducted a room-by-room search of the campus.
All schools in the area were locked down while police and fire crews responded to the scene, The Denver Post reported.
Some of the worst mass shootings in the US have taken place in Colorado.
In 1999, two pupils killed 13 people at Columbine High School before committing suicide in what remains one of the deadliest school shootings in US history.
In 2012, a man opened fire at a movie theatre in Aurora during a screening of a Batman film, killing 12 people and injuring scores more.
