The YouTube account was taken down sometime after 10:30 p.m. local time on Tuesday.
Sheriff Spurlock said neither suspect had been on law enforcement’s radar before the shooting and that the motive was unknown. He declined to say how the suspects had obtained the guns; in Colorado, it is illegal for anyone under 21 to own a handgun.
The shooting at the Highlands Ranch charter school is the latest at an educational institution, a phenomenon that has rattled communities nationwide as young people continue to face mortal danger in places long considered safe havens.
STEM School Highlands Ranch, which has about 1,800 students in kindergarten through 12th grade, will be closed for the rest of the week. Douglas County is an affluent area south of Denver with about 350,000 people. It sits next to Jefferson County, home to Columbine High School, and students there are already primed to watch for gunmen.
Mr. Castillo’s attempt to subdue the shooting suspect seemed to parallel the actions of Riley Howell, who took down the gunman at U.N.C. Charlotte last week. Both young men died and, according to the authorities, probably saved lives.
For many people, the notion of having young people tackle gunmen in the middle of a rampage is a chilling thought. But Greg Crane, the founder of an active-shooter training program that encourages people to “counter” shooters, said in situations where people cannot escape, disrupting the gunman might be the best approach to end a deadly tragedy.
“Frankly, this is the way these events get stopped the quickest,” said Mr. Crane, who created the “Alice” training method, which stands for Alert, Lockdown, Inform, Counter, Evacuate. “Waiting on law enforcement is not enough.”
Mr. Crane’s method has been criticized precisely because it urges civilians to take such an aggressive approach against gunmen. Other training approaches emphasize running or hiding and letting trained law enforcement confront the attacker.
“But if they don’t do anything and they maintain a static, passive position, waiting for the police to get there — as in Columbine, as in Virginia Tech, as in Sandy Hook — I think you see the casualty statistics are much higher,” Mr. Crane said.