The "juvenile female" joined Devon Erickson, an 18-year-old male, in shooting up STEM School Highland Ranch, the sheriff said.
Her name has not been released.
The sheriff's office posted the press conference footage on its Facebook page but the sound didn't work, prompting complaints from Facebook users. Another portion of the press conference was posted separately and the audio worked in that post.
Erickson was set to graduate and would have left the school in three days, Spurlock added. Deputies were able to apprehend both the male and the female without firing any shots.
The STEM campus is one block from the sheriff's office, enabling deputies to arrive in about two minutes.
“I believe the quick response of officers helped save lives,” Spurlock said. “A quick response eliminates a lot of this issue right off the bat.”
18th Judicial District Attorney George Brauchler said that the suspects are presumed innocent and that the adult suspect, Erickson, will be getting a publicly-appointed lawyer. He wasn't sure of the schedule for the girl.
"In our state, we have two tracks for adults and juveniles at this point in the process," he noted.
Michael Schwartz, one student, said that Erickson had discussed causing harm in the past.
"I always thought he was just messing around and stuff, but sometimes he did hint at it here and there," Schwartz said.
Nui Giasolli, another student, said that she was in her British literature class when Erickson walked in.
Other students piled on, helping Castillo tackle Erickson.
They “were brave enough to bring him down so that all of us could escape and all of us could be reunited with our families," she said. "I can’t thank them enough."
The father of Brendan Bialy, a senior who plans to become a Marine after graduating, said that his son was part of the group who stopped Erickson.
School district officials, meanwhile, announced on Tuesday night that the school would be closed for the rest of the week.