Denver (CNN) Chris Elledge should be looking forward to finishing his freshman year and enjoying summer break.

Instead, he and hundreds of other students are struggling to cope with the mass shooting that killed a young hero and wounded eight others at their Colorado school.

"It's just scary not knowing what's going to come next," the 15-year-old said.

"How I was feeling in the moment is too horrible to remember. And I can't imagine anyone who had to go through that in the past week or in the past year or whoever's had to go through a shooting like what happened here."

Now, newly obtained documents show the Douglas County School District had urged the STEM school to investigate allegations of violence and bullying that an anonymous parent feared could lead to "a repeat of Columbine."

A parent voiced concerns, and the district sued her

Five months before Tuesday's attack, a parent called the county Board of Education to express "concerns about student violence," according to a school district letter obtained by CNN.

The parent cited an alleged bomb threat and student clashes as evidence that the school could become the site of another Columbine-style attack, according to the letter. She also made about a dozen other allegations against the school, including a claim that "money is being sent to China and Mexico."

In response, Douglas County School District official Daniel Winsor wrote a letter to the school's executive director, Penelope Eucker. He asked her to investigate the parent's concerns, including claims that "many students are suicidal and violent in school. Several students have reported sexual assault in school and that nothing is being done."

"The concerns expressed by this individual are very serious and need to be looked into to the extent possible. Please keep (the district) apprised of your investigation and conclusions," Winsor wrote.

Eucker followed up with a letter to parents in February, disputing many of the anonymous parent's claims.

Her letter said allegations against the school's administration were investigated but could not be substantiated. But the letter did not address the concerned parent's claims that the environment could lead to severe student violence.

Instead, the letter said the district sued the anonymous parent, "Jane Doe," for spreading what they called "defamatory statements" about the school.

Court documents confirm this lawsuit was filed in January.

In a statement issued by a public relations firm, Eucker said the school's administrative team contacted 2,800 parents.

"STEM did not receive responses from any parents or students with information about these allegations," Eucker said. "While STEM took the allegations seriously, our investigation revealed no evidence to support any of the allegations raised in the anonymous complaint."

Winsor could not be immediately reached for comment. CNN has not been able to verify the parent's allegations, including the alleged bomb threat, with law enforcement.

Two suspects could be charged Friday

While trauma still blankets the Denver area, investigators are trying to understand why the attack happened.

Two suspects -- 18-year-old student Devon Erickson and 16-year-old Alec McKinney -- appeared in court for the first time Wednesday.

Devon Erickson

Erickson took the two handguns used in the shooting from his parents, said a law enforcement source with direct knowledge of the investigation. Both guns were purchased legally, the source said.

The suspects likely will be charged Friday with first-degree murder and attempted murder charges, 18th Judicial District Attorney George Brauchler said.

He said it's not yet clear whether the 16-year-old will be tried as an adult.

Authorities initially described the 16-year-old suspect as female. But the suspect's lawyer said in court that McKinney goes by the first name Alec and uses the pronoun "he," Colorado Judicial Department spokesman Rob McCallum said.

Hero student immediately jumped the killer

Kendrick Castillo was killed trying to rush one of the shooters, classmates said.

Brendan Bialy said he and Kendrick Castillo were sitting near a classroom door when someone from their class arrived late and took out a gun.

Without missing a beat, Castillo rushed the gunman.

"It was immediate, nonhesitation, immediate jump into action," Bialy said.

"The gunman was there, and then he was against the wall and didn't know what the hell hit him."

Bialy and a third student then joined in and wrestled with the shooter, who may have fired once or twice more during the struggle, Bialy said.

Once Bialy got the gun away and the other student had the shooter pinned down, he went over to Castillo, who wasn't responding.

Castillo, 18, died in the classroom.

The slain student was a kind, hilarious jokester who was always ready to help his classmates, student Tuscany "Nui" Giasolli said.

Nui's mother, Nyki Giasolli, said if not for the heroism of her daughter's classmates, "I wouldn't have my baby today."

"All these kids are alive because of (Castillo's) sacrifice and the bravery of all the boys to neutralize the threat," Giasolli said.

Student recalls shooter screaming, 'Shut up or I'll shoot all of you'

Vivaan Kalura, 12, was in math class when he and his classmates heard screaming and thudding.

"I'm pretty sure the shooter said, 'Shut up or I'll shoot all of you,'" he told "CNN Newsroom."

He said everyone in the class began to cry.

JUST WATCHED 7th grader describes what shooter said to him Replay More Videos ... MUST WATCH 7th grader describes what shooter said to him 01:18

Behind their locked classroom door, the students and their teacher could hear law enforcement officers and the shooter.

An officer said two people were shot, Vivaan recalled, "and then everyone just kind of lost it."

Vivaan said he is still scared by Tuesday's events.