DENVER (AP) — A teenager suspected in a shooting attack at a suburban Denver high school that left two students in critical condition appeared fascinated with previous mass shootings including Columbine and expressed neo-Nazi views online, according to experts.

Since December, Desmond Holly, 16, had been active on an online forum where users watch videos of killings and violence, mixed in with content on white supremacism and antisemitism, the Anti-Defamation League’s Center on Extremism said in a report.

Holly shot himself following Wednesday’s shooting at Evergreen High School in Jefferson County. He died of his injuries. It is still unclear how he selected his victims. The county was also the scene of the 1999 Columbine High School massacre that killed 14 people.

Holly’s TikTok accounts contained white supremacist symbols, the ADL said, and the name of his most recent account included a reference to a popular white supremacist slogan. The account was unavailable Friday. TikTok said accounts associated with Holly had been banned.

Holly’s family could not be reached. The Associated Press left a message at a telephone number associated with the house that police searched after the shooting.