"Not only have we run drills but our own experience with three school shootings and also the Douglas County deputy event, I'm afraid, makes us prepared, but you never get used to it", Elliott said. They were both allegedly armed with handguns but did not exchange gunfire with the officers who detained them.
"Avoid area", the Douglas County Sheriff's Office said in an alert on Twitter.
"We are going to hear about very heroic things that have taken place at the school", Spurlock said.
"That student was going to graduate and would have been out of school in three days", Douglas County Sheriff Tony Spurlock said of Castillo during a news conference Wednesday.
"When we were exiting the building there was a trail of blood", said an unnamed student.
A member of the school's robotics club and a relentless tinkerer, Castillo had an infectious smile and gentle sense of humor, according to friends.
A student at the school, Nui Giasolli, described how the gunman walked into her class on Tuesday afternoon, closed the door, brandished a gun and told everybody not to move. Castillo, 18, was killed when the suspects opened fire in a classroom at the K-12 charter school located south of Denver.
"The [STEM] school is literally two minutes away from where I live, right down Broadway in Highlands Ranch", Malone said.
A ranking law enforcement source told Denver7 that at least one victim was shot and that one suspect was down.
Authorities worked to obtain search warrants for the suspects' homes as well as a vehicle found in the school parking lot, Spurlock said.
There were around 1,800 students on campus at the time of the attack, Sheriff Spurlock said.
The father of the student killed in a Colorado school shooting says his son was a hero and he wants people to know about him.
On April 20, 1999, two gunmen killed 12 fellow students and a teacher in a mass shooting at Columbine.
"Who were are is, we are a kind, compassionate, caring people", Brauchler said, "and this does not define us".
The lockout was later lifted from Highland Ranch schools, the district said.
Elliott said he hopes this is the last school shooting his staff responds to, but they will continue to train and prepare. Spurlock said that the two were not apprehended together.
The sheriff's office directed parents to a nearby recreational center to pick up their children.
School shootings have since become a frequent occurrence in the United States, where firearms killed almost 40,000 people in 2017, according to the Centers for Disease Control and Prevention.