The young man who was slain has been identified as Kendrick Ray Castillo, a robotics enthusiast, the Douglas County sheriff's office said on Twitter.
Douglas County Sheriff Tony Spurlock said two shooters walked into the campus and opened fire on their classmates in "two separate locations".
According to the Douglas County Sheriff's Office (DCSO), authorities have arrested two suspects that are both male students.
Erickson will appear in court at 1:30 p.m. on May 8. His black hair, streaked with purple dye, covered his face as he nodded in response to most of District Court Judge Theresa Slade's questions.
"The thing that makes you angry is that, she's telling me how scared my daughters are in their schools, texting her, because they don't know what's going on - it's a lockout", Malone said of his conversation with his wife earlier on Tuesday.
A witness told CNN Castillo was shot while rushing one of the shooters.
The STEM School has about 1,800 students, the undersheriff said.
"Neither one of them were of legal age to own or purchase a gun", Spurlock said, when asked whether any "red flag" laws might have helped prevent the tragedy.
Marine Capt. Michael Maggiti said the teen was not injured.More news: North Korea fires several short-range missiles into the sea - South Korea
Police dispatch tapes recorded officers looking for one suspect with black hair wearing a Nirvana hoodie and one with purple and black hair and a black hoodie.
The other suspect has been identified by police as 18-year-old Devon Erickson.
"We originally thought the juvenile was a male by appearance", Spurlock said.
Two Columbine students killed 13 people there in 1999 before committing suicide in what remains one of the deadliest school shootings in United States history. "They completely and utterly failed in a matter of half a minute", he said of the attackers. The station cited anonymous sources close to the investigation. A sheriff's department substation is nearby and authorities responded quickly and in force.
- The father of a student killed in a Colorado high school shooting on Tuesday said his son was a hero and he wanted people to know about him. As he charged, so did two other students. "I share the heartbreak, the frustration, the sickness".
The shooting occurred about eight miles from Columbine High School where 12 students and a teacher died in a mass shooting 20 years ago.
It also comes almost three weeks after the 20th anniversary of the Columbine school massacre that killed 13.