Douglas County District Attorney George Brauchler says 18-year-old Devon Erickson will make an initial appearance at 1:30 p.m. Wednesday.
Messages reportedly sent from a pupil in the school, and shared on social media, said "some seniors tackled the gunmen" before adding there was "blood on the floor".
Nui Giasolli, another student, said that she was in her British literature class when Erickson walked in. Police struggled with them to take them into custody.
A source told Denver7 that investigators were using a bomb robot near one of the suspect's vehicles because tactical gear was found inside.
On April 20, 1999, two students killed 12 of their classmates and one teacher in a mass shooting at Columbine High.
Police were at the school within two minutes of the shooting, according to reports.
Two of the victims remain in serious condition at Littleton Adventist Hospital while three others were released.
The Colorado attack unfolded came almost three weeks after neighboring Littleton marked the anniversary of the Columbine attack that killed 13 people. "The next thing I know he's pulling a gun and he's telling nobody to move", she said Wednesday.
"For it not to be safe in a kid's school is tragic", Nuggets guard Jamal Murray said after Tuesday's game.
Though authorities initially said they were looking for a possible third suspect, Nicholson-Kluth said they are no longer looking for another suspect.
Spurlock said investigators can only say at this point that a handgun was used.
Parents fight back tears as they wait for their children at a recreation center where students were reunited with their parents in Highlands Ranch, Colo. on May 7, 2019.
The attack came a week after a gunman opened fire on the Charlotte campus of the University of North Carolina, killing two people and wounding four others.
Authorities on Tuesday identified the juvenile suspect as a boy but Spurlock on Wednesday told reporters that the juvenile suspect is a girl.
A day after the shooting at the STEM School in Highlands Ranch, Colo., local officials urged people not to view such attacks as part of life in Colorado.
Media captionHow much do U.S. students fear school shootings?
Polis also thanked first-responders, who he said "prevented an even worse tragedy from unfolding".
Police dispatch tapes recorded officers looking for one suspect with black hair wearing a Nirvana hoodie and one with purple and black hair and a black hoodie.