The gunshots could still be heard as deputies ran toward the STEM School Highlands Ranch in Colorado.
Two suspects, believed to be students at the school, had made their way inside Tuesday and were shooting inside two classrooms.
Within minutes, an 18-year-old student was dead and eight students were wounded.
It was a scene disturbingly familiar for this suburban Denver community, which just marked the 20th anniversary of the Columbine High School shooting in which two students opened fire on their classmates, killing 12 of them and a teacher.
The Columbine shooting changed the way police respond to active shootings. In 1999, it took 47 minutes after the gunfire erupted for SWAT teams to enter the Littleton school. On Tuesday, authorities were on the scene and inside the STEM school mere minutes after getting the first calls.
Two suspects were taken into custody, one of them identified by the Douglas County Sheriff's Office as 18-year-old Devon Erickson. The other suspect is a juvenile.
"I have to believe that the quick response of the officers that got inside that school helped save lives," Douglas County Sheriff Tony Spurlock said.
The school will be closed the rest of the week.
Students tried to stop one of the gunmen
"(With) everything at Columbine, you never think this would be the reality of this situation," one mother of twins at the school told CNN affiliate KDVR.
The mother, who was not identified, recalled getting a terrifying phone call from her daughter.
"'Mommy, there's a gun (being) shot at my school,'" she remembered her saying.
Brad Bialy's son Brendan is a senior at the STEM school, which focuses on science, technology, engineering and math. Brendan told his father that the shooting started after the two suspects entered the classroom and one pulled a gun from a guitar case.
Brendan and two friends tried to tried to tackle one of the gunmen and one of the boys was shot in the chest, Brad Bialy told the New York Times.
Other students put pressure on the wound. The condition of the boy shot was not known.
In a message to parents, Spurlock said police will do "everything we can to investigate this" and determine what happened.
"This is a terrible event," he said. "This is something that no one wants to have happened in their community."
Fernando Montoya's 17-year-old son was shot three times. He was released from the hospital Tuesday night, Montoya told CNN affiliate KMGH. Montoya knows how lucky he is.
"Thank God he is fine," Montoya said. "Even though he got shot, he's OK. He's going to walk out on his feet."
He's still in disbelief.
"It's hard to believe that someone in the classroom is going to have a weapon and is just going to open fire like nothing," he told the news station.
Spurlock said investigators can only say at this point that a handgun was used. Authorities are expected to release more information at a news conference Wednesday morning.
A community targeted before
The STEM school is just seven miles away from Columbine High School.
"Tragically, this community and those surrounding it know all too well these hateful and horrible acts of violence," the White House said in a statement.
Douglas County Schools were also 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.
More than 1,850 students attend the STEM school, which includes grades K-12, according to its website. Spurlock said the shooting took place in the high school.
The school did not have an assigned school resource officer but did have private security, according to Spurlock.
"Our STEM family is hurting tonight. Our DCSD family is hurting tonight," Douglas County School Superintendent Thomas S. Tucker said in a letter to families. "Supporting one another in the coming days, weeks and months is critical."
The letter, Tucker said, is one he hoped he'd never have to send.
Search warrants for suspects' homes and vehicle
Investigators searched a home Tuesday night in Highlands Ranch related to the STEM school shooting, Cocha Heyden, a spokeswoman for Douglas County, told CNN.
Officials would not confirm if one of the shooters lives at the home.
Authorities worked to obtain search warrants for the suspects' homes as well as a vehicle found in the school parking lot, Spurlock said.
He did not release additional information on the suspects. The two didn't appear to be on law enforcement's radar, Spurlock said.