Douglas County Undersheriff Holly Nicholson-Kluth said the victims, who are all over 15 years, had gunshot wounds and were taken to three local hospitals.

The Denver Post, a local newspaper, reported that at least four had been treated and released from hospitals by Tuesday evening.

Nicholson-Kluth briefed reporters that the officers heard gunfire as they arrived and as they entered the building, saying "quite a few shots were fired."

The incident occurred at STEM School Highlands Ranch, located about 32 km south from downtown Denver. There are about 1,850 students in the K-12 charter school.

After hearing reports of the shooting, worried parents raced to the school to search for their children. They were redirected to a nearby recreation center to wait for reunions.