An 18-year-old was killed after two suspects opened fire at a high school in suburban Denver on Tuesday, authorities said.

"It is with extreme sadness that we can confirm that 1 student at the STEM School was killed in today's #stemshooting incident. The immediate family has been notified. Douglas County Coroner Jill Romann has not officially identified the student is stating it's an 18 year old male," Douglas County Sheriff's Office wrote on Twitter.

Meanwhile, the two suspects were arrested after the shooting at STEM School in Highlands Ranch, which has over 1,800 students. Both the accused are students -- one is an adult male and the other is a juvenile, CNN quoted Douglas County Sheriff Tony Spurlock as saying.

The shooting began just before 2 pm (local time) and the suspects "got deep inside the school" and opened fire at the students at two separate locations, Spurlock said.

School authorities informed the police as soon as the incident unfolded. The students were immediately evacuated to a safer area.

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

Spurlock informed that the first sheriff's deputies arrived within two minutes of receiving a distress call from the school and engaged with the accused persons.

He said that the police "did struggle with the suspects to take them into custody." The names of the suspects were not revealed, according to CNN.

Nicholson-Kluth confirmed that there was no third suspect in the shooting.

"We do not have any other suspects. We have all the people who were involved (in the shooting)," Spurlock said.

Following the shooting, the area surrounding STEM School was placed under lockdown. All schools in Highlands Ranch were also on a lockout. The schools re-opened after the situation stabilised.

Spurlock said several students were in a critical condition, with the youngest victim aged 15 years.

The injured students are being treated at three local hospitals.

While five of them are at Littleton Adventist Hospital in Littleton, a victim was being treated at Children's Hospital Colorado South Campus in Highlands Ranch and is said to be in a good condition.

Two juvenile students are being treated for their injuries at Sky Ridge Medical Center in Lone Tree.

(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)