One student is dead and eight students were injured in a shooting at a Colorado school on Tuesday, according to authorities.
The deceased male, who has not been identified, was 18 years old.
Two suspects are in custody, Douglas County Undersheriff Holly Nicholson Kluth told reporters during a press conference. Emergency dispatchers received reports that shots were fired at the STEM School in Highlands Ranch just before 2 p.m., according to a tweet by the sheriff's office.
Investigators believe one of the suspects is an adult and the other is a juvenile, and they are both students at the school, Douglas County Sheriff Anthony Spurlock told reporters.
The adult was identified late Tuesday as Devon Erickson, 18. Officials said they would not be releasing a photo of him "as it could jeopardize this critical ongoing" investigation.
Neither suspect was previously on the "radar" of law enforcement, Spurlock said.
Authorities do not have any information as to whether any of the injured were targeted, Spurlock said.
A struggle may have ensued between the suspects and someone at the school, Kluth said. The male suspects were not injured, Spurlock said.
(MORE: The 11 mass deadly school shootings that happened since Columbine)The suspects got "deep inside the school" and engaged students in two separate locations, Spurlock said, adding that one of the weapons used was a handgun. Investigators believe they have located all suspects involved, he added.
The injured students, who are all at least 15 years old, were taken to area hospitals, some in critical condition, Spurlock said. Some students have since been released, and have been reunited with their families, he added.
The school does not have a school resource officer, but the school does have private security, Kluth said. School staff notified emergency dispatchers "almost immediately" after the first gunshots were fired, and deputies were on scene within two minutes and engaged the suspects, Spurlock said.
The charter school teaches kindergarten through 12th grade and has more than 1,800 students, Kluth said. More than 500 of those students are elementary age, ABC Denver affiliate KMGH reported.
(MORE: 6th grade students charged with plotting Tennessee school shooting after rumors of 'hit list' circulated, officials say)The school was under lockdown as SWAT teams went from classroom to classroom to clear them, Kluth said. Nearby schools were placed on lockout, according to KMGH.
"This is a terrible event," Spurlock said. "This is something that no one wants to have happen in their community."
Parents were instructed to pick up their children nearby at the Northridge Recreation Center. Images showed dozens of children in a single file line with their hands on their heads.
(MORE: UNCC instructor who escaped school shooting says students were 'running for their lives')Concerned mother Kelley Paulson told KMGH that her sixth-grade son got out of the school but her third-grade daughter was still inside.
The school was surrounded by first responders, Paulson said.
Kelly’s son, Christian, said he was in study hall when he saw a “bunch of kids running out and saying, ‘school shooter, school shooter.’” At first, he wasn’t sure the situation was real, he said.
(MORE: 20 years after Columbine, what's changed -- and what hasn't -- for school shootings in America)“I tried to run with my life but I’m out of breath,” Christian said.
Colorado Gov. Jared Polis said the state is making all public safety resources available to assist the sheriff's office.
"The heart of all Colorado is with the victims and their families," Polis said in a statement.
ABC News' Malka Abramoff, Bonnie Mclean and Alyssa Pone contributed to this report.