"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.
"We know two individuals walked into the STEM school, got deep inside the school and engaged students in two separate locations", Douglas County Sheriff Tony Spurlock said at a news conference. "The next thing I know he is pulling a gun and is telling nobody to move", she said.
Bialy would not identify the third student who helped subdue the gunman, but the family of Jason Jones put out a statement saying he was shot twice while disarming one of the attackers.
Student Michael Schwartz said Erickson, in the past, had spoken about hurting other people.
Details are emerging about the lives of the two suspects.
"As officers were arriving at the school they could still hear gunshots", Nicholson-Kluth said.
Spurlock declined to answer a reporter's question about whether the younger suspect was transgender.More news: Here's when we'll see Baby Sussex for the first time
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"When we were exiting the building there was a trail of blood", said an unnamed student.
"We originally thought the juvenile was a male by appearance", Spurlock told AP.
Castillo, she said, didn't hesitate. The station cited anonymous sources close to the investigation. Eight other students were injured in the shooting.
Other students confirmed Giasolli's account to CBS News. "I share the heartbreak, the frustration, the sickness".
The state had just commemorated the 20th anniversary of the notorious Columbine school shooting in which two heavily armed students aged 17 and 18 killed 12 of their classmates and a teacher on April 20, 1999, before committing suicide.
Two surviving victims of the attack at the Science, Technology, Engineering and Math (STEM) School in Highlands Ranch, a Denver suburb, remained in serious condition, medical officials said. At the University of North Carolina at Charlotte last week, a student tackled a gunman who opened fire in a classroom, police said, killing two and wounding four others.
But what happened inside STEM remains unclear.
"All of a sudden a guy pulled I believe a pistol, I'm not sure, out of a guitar case and start to shoot", Montoya said.