(CNN) -- Eight students were injured Tuesday in a shooting at a school in suburban Denver, authorities said.
Two suspects were apprehended after the shooting at the STEM School Highlands Ranch, which covers K-12, Douglas County Sheriff Tony Spurlock said. The suspects are students; one is believed to be an adult male and one is believed to be a juvenile, he said.
The shooting started in the high school just before 2 p.m., authorities said. The suspects walked into the school, and "got deep inside the school," engaging students in two separate locations, Spurlock said.
Almost immediately, the school notified authorities.
"Over the next few minutes, quite a few shots were fired," Douglas County Undersheriff Holly Nicholson-Kluth said.
"As officers were arriving at the school they could still hear gunshots," Nicholson-Kluth said.
The first sheriff's deputies arrived within two minutes and engaged the suspects, Spurlock said.
Spurlock said officers "did struggle with the suspects to take them into custody."
He did not release the suspects' names.
Several victims are critical, the sheriff says
Though authorities initially said they were looking for a possible third suspect, Nicholson-Kluth said they are no longer looking for another suspect.
"We do not have any other suspects.. We have all the people who were involved," Spurlock said.
A number of students were shot and several are in critical condition, the sheriff said. The youngest victim is 15 years-old, he said.
At least three local hospitals were treating eight patients after the shooting.
Five patients were being treated at Littleton Adventist Hospital in Littleton, hospital spokeswoman Wendy Forbes told CNN in an email.
One victim was in good condition at Children's Hospital Colorado South Campus in Highlands Ranch, according to Jaime Berg, a spokeswoman.
Two juvenile patients were being treated at the Sky Ridge Medical Center in Lone Tree.
The STEM School was placed on a lockdown after the shooting, the Douglas County School District said in a tweet. All other Highlands Ranch schools were on a lockout, the tweet said.
The lockout was later lifted for Highlands Ranch schools, the district said.
"This is a good community. It's usually very quiet, extremely low crime rate," said the undersheriff, who has been with the department 30 years.
The STEM School has about 1,800 students, the undersheriff said. It is located about seven miles from Columbine High School, which is in Littleton.
On April 20, 1999, two students killed 12 of their classmates and one teacher in a mass shooting at Columbine High.
Douglas County Schools were closed April 17 as authorities scrambled to find an armed Florida teen they said was infatuated with the Columbine massacre. The woman, authorities said, made threats before she traveled to Colorado, where she died of a self-inflicted gunshot wound.