The fourth mass school shooting in the United States so far this year wounded eight students, some of them seriously, on Tuesday afternoon at the STEM School Highlands Ranch at Douglas Country, Colorado. No one was reported killed and the two gunmen were later arrested by police.

The names of the wounded students and the two suspects have not been released.

The Douglas County Sheriff's Office said it immediately responded to the shooting, which took place just before 2:00 p.m., when a school administrator called dispatch to report shots fired.

"Two individuals walked in to the STEM school, got deep inside the school and engaged students in two separate locations," said Douglas County Sheriff Tony Spurlock. The first sheriff's deputies arrived within two minutes and engaged the suspects, said Spurlock.

He added one suspect was a male teenager while the other was an adult male. Both are students, he added. Spurlock also said several of the wounded students are in critical condition.

A police spokeswoman said they had no further information about the suspects right now.

Undersheriff Holly Nicholson-Kluth said county police heard shots as they were entering the school.

"Over the next few minutes, quite a few shots were fired," said Holly Nicholson-Kluth. "As officers were arriving at the school they could still hear gunshots.”

Multiple agencies, including the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives, also responded to the shooting.

The school is located just eight miles southeast of Columbine High School in Littleton, site of the April 20, 1999 shooting where two gunmen murdered 12 fellow students and a teacher before killing themselves.

The shooting is also the first in Colorado since Dec. 13, 2013 when a male student at Arapahoe High School shot and killed a female student. The shooter later took his own life.

STEM School Highlands Ranch has 1,850 students (550 elementary age students, 700 in middle school and 600 high school students), said the school district.

At least three local hospitals are treating the eight wounded students.

Colorado school shooting A woman walks out of the Recreation Center at Northridge with her son after at least seven students were injured during a shooting at STEM School Highlands Ranch on May 7, 2019 in Highlands Ranch, Colorado. Photo: Michael Ciaglo/Getty Images

Five patients are at Littleton Adventist Hospital in Littleton. One victim is in good condition at Children's Hospital Colorado South Campus in Highlands Ranch. Two more injured students are being treated at the Sky Ridge Medical Center in Lone Tree. The youngest victim is 15 years-old.

STEM School Highlands Ranch was placed on a lockdown after the shooting, tweeted the Douglas County School District. All other Highlands Ranch schools are also on a lockout. The lockout was later lifted for Highlands Ranch schools.

"This is a good community,” said Nicholson-Kluth. “It's usually very quiet, extremely low crime rate.”

Before the Colorado shooting, two people were killed and six injured in three school shootings in 2019, according to NBC News research. Six other shooting incidents took place at U.S. schools and involved students and teachers but weren’t mass shootings.

First responders at Colorado school shooting First responders stage on a road outside STEM School Highlands Ranch where at least seven students were injured during a shooting on May 7, 2019 in Highlands Ranch, Colorado. Photo: Michael Ciaglo/Getty Images