The two students accused of opening fire on classmates at their Denver high school have been revealed as an 18-year-old boy and a young girl - as authorities say the 18-year-old student who was shot dead was due to graduate in just three days.

Devon Erickson, 18, and a juvenile female - who authorities previously identified as a boy - were taken into custody following the shooting at STEM School Highlands Ranch in Denver on Tuesday afternoon.

Erickson was charged as an adult on Wednesday with first-degree murder and attempted murder ahead of an initial court appearance in the afternoon. His accused accomplice is being charged separately as a juvenile.

The Douglas County Sheriff's Office said they had initially believed the female shooter was a male based on appearances and were not able to confirm her gender until an interview had taken place.

The underage student is reportedly in the process of transitioning from female to male but Sheriff Tony Spurlock would not confirm if the young suspect was transgender.

'Right now we are identifying the individual as a female, because that's where we're at,' he said. 'We originally thought the juvenile was a male by appearance.'

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Devon Erickson, 18, has been identified as one of the two suspects who allegedly opened fire on STEM School Highlands Ranch in Colorado on Tuesday afternoon

Kendrick Castillo, 18, was killed when the two students stormed into two classrooms and opened fire with handguns that were concealed in a guitar case.

Eight students suffered gunshot wounds in the shooting and three of them remain in intensive care in hospital. Five had been discharged from the hospital by Wednesday morning.

Authorities have not yet released a motive for the shooting, which occurred just miles from the scene of the deadly 1999 Columbine High School massacre.

Kendrick Castillo, 18, died on Tuesday afternoon when two students stormed STEM School Highlands Ranch in Denver and opened fire with at least two handguns

Castillo, the 18-year-old student who was shot dead, was just three days away from graduating, according to Douglas County Sheriff Tony Spurlock.

Classmates said Castillo was among a group of three students who tried to take down one of the shooters by tackling them when they stormed the classroom.

The father of one of those heroic students, Brendan Bialy, told the New York Times that the trio tackled the shooters when they entered the classroom 'and one pulled a gun out of a guitar case'.

He said 'his son and two friends tried to tackle the gunman but one of the boys was shot in the chest'.

'We're going to hear about very heroic things that have taken place at the school,' Sheriff Spurlock said.

The two suspects opened fire in two separate classrooms in the high school section of the campus, which serves grades kindergarten through 12th.

Authorities said they went into the school through an entrance without metal detectors.

They were arrested within minutes after administrators reported the gunfire, according to Sheriff Spurlock.

Spurlock said officers engaged the suspects and there was a struggle to bring them down but neither suspect was injured in the confrontation.

Authorities recovered three handguns and a rifle but said the latter was not used in the shooting. The two students were not old enough to buy or own the handguns.

Terrified students were evacuated from the school after shots were fired at 1.53pm

Fernando Montoya (above with his daughter) said that his 17-year-old son was shot three times but was expected to make a full recovery

Authorities were seen outside a home in Highland Springs believed to be owned by Erickson's parents, James and Stephanie, on Tuesday night

Car towed from #stemshooting suspect's home apparently has "F*** SOCIETY" spray painted on the side. Also "666" and a what looks like a pentagram sprayed on the hood. pic.twitter.com/e6QX3lq4v3 — John Fenton (@higuysimjohn) May 8, 2019

Douglas County Sheriff Tony Spurlock said Devon Erickson, 18, and a younger student walked into the STEM School Highlands Ranch through an entrance without metal detectors and opened fire with two handguns on students in two classrooms

Authorities were seen outside a home in Highland Springs believed to be owned by Erickson's parents, James and Stephanie. The mother's Facebook page was deleted shortly after news broke of the shooting.

A car spray painted with the words 'f*** society' was towed away from the house at around 10pm.

A criminal background check revealed Erickson's almost unblemished record, which noted a ticket for careless driving issued in Douglas County on February 13, 2018.

His mother broke down in tears on Wednesday and said she didn't know why her son opened fire on classmates, the New York Post reports.

The teen's social media pages indicate that he is an avid guitar player, youth theater actor and registered Democrat who has on occasion expressed disdain for Christians, criticized President Donald Trump and praised former President Barack Obama.

In a post from 2014, Erickson wrote: 'You know what I hate? All these Christians who hate gays, yet in the bible, it says in Deuteronomy 17:12-13, if someone doesn't do what their priest tells them to do, they are supposed to die. It has plenty of crazy stuff like that. But all they get out of it is 'ewwwwww gays'.'

In 2015 he shared a Facebook post by Occupy Democrats praising Obama, and in 2016 shared a video of TV host Seth Meyers criticizing Trump.

Erickson also has a YouTube channel where he posted videos of himself playing the guitar.

His Snapchat account is reportedly under the name 'devonkills'.

In one Instagram post obtained by Heavy.com, Erickson wrote: 'I'm covered in ink and addicted to pain.'

In another from 2018 he said: 'I'm only posting one of my senior pictures so people don't think I dropped out since I don't take classes at stem anymore.'

A combination of factors, including revenge and anger, spurred the attack, the Denver TV ABC affiliate reported, citing law enforcement sources. One of the suspects had reportedly been bullied over gender identity.

Erikson was charged on Wednesday as an adult with murder and attempted murder

The teen's social media pages indicate that he is an avid guitar player and a youth theater actor. Erickson also has a YouTube channel where he posted videos of himself playing the guitar

The harrowing texts a teen sent his father as two shooters stormed his Denver high school The father of one of the Denver high school students forced to hide in his classroom when two shooters opened fire has shared the harrowing text messages he received from his teenage son as the gunfire rang out. Eldon Elledge's teenage son immediately texted his father when the alarms sounded and they were placed on lockdown at STEM School Highlands Ranch in Denver on Tuesday afternoon. His terrified son's messages became increasingly frantic as he told his father he could hear gunshots, was hiding in a classroom that had four separate entrances and that he loved him 'just in case'. Elledge shared the messages on Facebook on Wednesday and described it as 'the scariest moment of my life'. 'Dad the school is on an unexpected lockdown. They've got sirens playing all in the school and the radios are going crazy,' his son said in his first message. 'I'm in the P.E. room with four different entrances. Everyone is freaking out. It's not a drill I'm actually scared. My heart is racing. I'm actually freaking out, I'm really scared.' Elledge replied to his son and pleaded with him to remain calm and 'remember your training' for an active shooter situation. 'Dad the announcements are going crazy. The speakers are telling everyone to get out of sight. The teacher thinks somebody is in the school,' his son wrote. Elledge told his son he was on his way to the school, adding: 'But remain calm, breath slowly and pay attention. Listen to all sounds. Breath short quite breathes and listen to anything off.' His son's messages became increasingly more terrified as the teenager described what was unfolding inside. 'Okay. There is a bunch of yelling in the hallway,' he wrote. 'There is gunshots. Dad. Gunshots. Oh my god. The school is flooded with police. Gunshots. 'Dad there is no where to hide, it's an open room. 'All I hear is yelling and screaming. I can't call the teacher won't let me. What do I do dad I'm freaking out. The screaming has stopped. 'Dad I love you, just in case.' Elledge told his son he loved him as well before adding that a lot of children were coming out of the school and repeatedly asking where he was. 'Somebody is dead. Somebody died,' his son wrote replied. Advertisement

Fernando Montoya told television station KMGH that his 17-year-old son was shot three times but was expected to make a full recovery.

'Thank God he is fine,' Montoya said. 'Even though he got shot, he's OK. He's going to walk out on his feet, so I'm glad. We're so lucky.'

As gunfire echoed through the school, students hid or ran through the halls. Students ran through the halls shouting 'School shooter!' and some wondered at first if it was a joke or a drill.

'No one really knew what was going on so I didn't know they were bullets,' said seventh-grader Sophia Marks. 'I just kind of saw like flashes and we heard bangs.'

Student Nui Giasolli told the Today show she was in her British Literature class when Erickson came in late.

'He walked to the other side of the classroom where we also had another door and he opened the door. He walked back as if he was going to go back to his seat, then he walked back to the door and he closed it. The next thing I know he's pulling a gun and he's telling nobody to move,' she said.

Giasolli said another student lunged at the shooter, giving the other students time to dive under the desks and flee to safety.

'To be some of the kids that were brave enough to bring him down so that all of us could escape and all of us could be reunited with our families. They were very heroic. I can't thank them enough.'

Chris Elledge, 15, said his teacher told the class to hide behind weight equipment in the room, where they stayed until police arrived.

'They busted in the room, and they were asking if there was any suspects in the room, if we were OK, and they escorted us out to go out to the front of the building,' Elledge said.

Josh Dutton, 18, told The Associated Press that he was close friends with Devon Erickson in middle school but hadn't seen him for four years as he went to a different high school.

On Sunday, he spotted Erickson at a local light rail station and said he was shocked at how much his friend had changed.

Erickson wore all black, a hat and sunglasses, was significantly skinnier and didn't seem interested in talking.

'He said he'd just turned 18 and he owned rifles,' Dutton said.

A teacher embraces young students in a parking lot near the STEM School Highlands Ranch in Colorado on Tuesday after a shooter opened fire

Parents embrace their children as they are reunited at a recreation center near the school

Students and teachers hold their hands in the air as they exit the scene of the shooting

A massive police presence descended on the school minutes after shots were fired at 1.53pm

Parents pick up their children from the Recreation Center at Northridge in Highlands Ranch after the shooting at the STEM School Highlands Ranch on Tuesday

Hundreds of people took to Twitter after news of the shooting broke to express their horror that yet another American school had been targeted.

The shooting is the 43rd incident of gunfire reported on a school campus so far in 2019 and the 116th mass shooting overall this year.

The STEM campus is located just seven miles from Columbine High School, where 13 people were killed and more than 20 were injured on April 20, 1999 - just over 20 years ago.

One student at a nearby high school tweeted: 'Being a student in Colorado is terrifying. Threats, secure permitters, lockdowns, missing school because of credible threats, and now another school shooting. I'm tired. What is the government doing to help us? My death is inevitable. Please talk about this.'

Another user tweeted: '20 years later, 7 miles from Columbine, and 0 actions taken in Washington to protect kids from gun violence.'

The White House issued the following statement about the shooting: 'Our prayers are with the victims, family members, and all those affected by today's shooting at STEM School Highlands Ranch in Littleton, Colorado. Tragically, this community and those surrounding it know all too well these hateful and horrible acts of violence.

'The White House has been in communication with state and local officials, and the President has been briefed and continues to monitor the ongoing situation. We offer our full support to local law enforcement and first responders and thank them for their heroism.'