Pictures posted online showed hundreds of parents crowded at the Northridge Recreation Center in Highlands Ranch, where students from the school were taken there by buses after the fatal incident. Many parents broke into tears when they found their children.

Colorado Governor Jared Polis said in a tweet that the state is making an all out effort to assist the Douglas County Sheriff's Department secure the site and evacuate the students.

"We are monitoring the situation in real time. The heart of all Colorado is with the victims & their families," he tweeted.

After the shooting, US President Donald Trump's daughter, Ivanka, cancelled her scheduled Wednesday's trip to the Lockheed Martin facility in Littleton, 12 km north of Highlands Ranch.