First responders gather after a shooting at a STEM School Highlands Ranch in Highlands Ranch, Colo., May 7, 2019.

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.

The suspects are believed to be juveniles, but it is unclear whether they are students at the school, Kluth said. A struggle may have ensued between the suspects and someone at the school, she added.
 

Multiple people who were injured were students, Kluth said. The extent of their injuries is unclear.

The school does not have a school resource officer, but a school resource officer in the area was one of the first authorities on the scene, Kluth said. It is still an active shooter situation.
 

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.

The shooting started in the middle school, Kluth said.

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.

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.
 

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.

“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.

Additional information was not immediately available.

This is a developing story. Please check back for updates.