DENVER – 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.

A spokesperson for the Jefferson County Sheriff’s Office, Mark Techmeyer, declined to comment on the ADL’s findings or discuss its investigation into the shooting. The office previously said that Holly was radicalized by an unspecified “extremist network” but released no details.

Two recent suspects in school shootings were active on the so-called “gore forum” that Holly used — Watch People Die, according to the ADL. Holly appears to have opened his account in the month in between shootings in Madison, Wisconsin and Nashville, Tennessee, the ADL said.

A few days before Wednesday’s shooting, Holly posted a TikTok video posing in a similar way to how the Wisconsin shooter posed before killing two people during in December. He included a photo of the Wisconsin shooter in a post in which Holly wore black T-shirt with “WRATH” written on the front.

He also posted videos showing how he had made the shirt that was like one worn by a gunman in the Columbine shooting, the ADL said.

“There is a through-line between those attacks,” said Oren Segal, the ADL’s senior vice president of counter-extremism and intelligence. ”They’re telling us there is a through line because they are referencing each other.”

Emails sent to Watch People Die seeking comment weren’t immediately returned.

Holly was also active on TikTok’s “True Crime Community,” where it says users have a fascination with mass murderers and serial killers, the ADL said.

Some TikTok posts shared by the ADL show one user encouraging Holly to be a “hero,” a term it says white supremacists use to refer to successfully ideologically motivated attackers.

The person also told Holly to get a patch with a Nazi-era symbol that was worn by the men who carried out the 2019 attack on a mosque in Christchurch, New Zealand, and the 2022 attack on a supermarket in Buffalo, New York.

Holly posted a photo of two patches that he had but said the Velcro on the back had fallen off.

“I’m gonna use stronger glue when I fix it,” he said.

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Brown reported from Billings, Montana.