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(CNN) - As if losing your child in a senseless school shooting isn't devastating enough, imagine how difficult it must be as a parent to hear there's a chance it could have been prevented.
A security guard at Arapahoe High School in Colorado has come forward, blaming the school district for ignoring several red flags leading up to last month's shooting. Police say Karl Pierson opened fire on the school in Centennial, Colorado before turning the gun on himself – 17-year-old Claire Davis was killed in the rampage.
Pierson may have been targeting a school librarian, as revenge for being disciplined, CNN has learned. In an interview with CNN affiliate KUSA, school security guard Cameron Rust revealed that Pierson had been on the school's radar long before the shooting for threatening a teacher, and even looking up guns on his computer.
Rust says school administrators were warned about Pierson's behavior, but didn't act.
"One afternoon in the lunchroom, Carl was looking up guns on his computer. And after we noticed that, he quickly closed his computer and we immediately told our supervisor what we saw, and we were told he would bring it up in their administration meeting," Rust told KUSA.
After the meeting, "we were told there was nothing they could do about that, and it was his personal computer, and he could look up what he wanted," said Rust.
"What's stopping us from doing more, to help and prevent this?" said Rust.
CNN reached out to the Littleton Public School District for comment, but has not heard back.
In an e-mail to parents, the district alluded to Rust's claims: "Much of the conversation is based on inaccurate information, rumor, and innuendo. I would encourage all warriors (the school mascot) to wait for the sheriff's investigation to come to a close before drawing conclusions."
Arapahoe County Sheriff Grayson Robinson told CNN in a statement that they interviewed Rust and plan to re-interview him in the future.