The shooting was confirmed by the Douglas County Sheriff's Office, which tweeted that the situation was "unstable" on Tuesday afternoon. Two people were arrested in what was described as a "unified" response to a shooting situation just miles away from Columbine High School where one of America's worst mass shootings shocked the US 20 years ago last month.
“We had school resource officers … enter the school almost immediately," Holly Nicholson-Kluth, the undersheriff in Douglas County, said during a press conference, noting that multiple agencies were involved and that the shooting began in the middle school.
Ms Nicholson-Kluth said that there was a struggle between individuals in the school and the shooter, but that law enforcement was not sure who had confronted the shooter.
A later update from Douglas County Sheriff Tony Spurlock indicated that two shooters were suspected of being involved, including one who was thought to be an adult male and another a juvenile male. Several of the students in hospitals, he said, are in "critical" condition.
The STEM school where the shooting reportedly occurred went into lock down after the incident, and parents were directed to pick up their kids at a nearby elementary school. Police said that some students had self-evacuated after the shooting began.
The school has students ranging in age from 5 years old to 18 years old.
"I got a text from a friend who was actually in there," Kelly Paulson, the mother of two students at the school, told Denver7. "She said 'guns, shooting, oh my god, oh my god.' And she could hear them and that's how I first knew. The next thing I know, I heard my son, who is calling me because all of the kids who were in middle school...all immediately ran out of the building."
Some of the students who were able to leave the school after the shooting began fled to a local brewery nearby, while one student who was injured during the attack was reportedly treated at a house down the street.
One mother of a 13-year-old student, speaking to local media, said that she had spoken to her son and that he told her he saw a "body and a trail of blood".
Senator Cory Gardner, who represents the state in Washington, tweeted shortly after the shooting first began.
"I am closely monitoring the situation at the STEM School in Highlands Ranch. My prayers are with the students, parents and faculty members and I’m grateful to the first responders working to keep everyone safe," Mr Gardner tweeted.
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