On the day he carried out the Columbine High School massacre, 18-year-old shooter Eric Harris wore a white T-shirt with the phrase “Natural Selection” printed on it in black lettering. His fellow assailant, 17-year-old Dylan Klebold, wore a black T-shirt with the word “Wrath” printed in red across the chest.

Ever since, this apparel has been closely linked with mass murder, part of an ever-growing mosaic of niche allusions to deadly attacks on schools, places of worship, government buildings, and social gathering spaces. While you might not think twice about a person wearing one of the shirts Harris and Klebold chose for their assault on their classmates, let alone recognize the reference, there’s an online community that revels in this aesthetic, using it as a means to valorize and motivate nihilistic violence.

According to a report publishing later this week from the Institute for Strategic Dialogue, a research organization that analyzes extremism and online threats, statement shirts associated with such horrific acts are widely available from at least half a dozen internet retailers.

ISD found sellers peddling “Wrath” and “Natural Selection” tees modeled after the Columbine shirts on TikTok Shop, Redbubble, Temu, Etsy, AliExpress, and Walmart. Shirts with the slogan “Humanity Is Overrated”—like the one worn by a teen gunman who killed nine people at his high school in Finland in 2007—turned up on AliExpress, Temu, and the TikTok Shop, according to the report.

In a statement, a representative for TikTok US said: “Discriminatory and offensive products are prohibited from sale on TikTok Shop and will be removed.” A representative for Redbubble said that the marketplace prohibits “content that glorifies or trivializes violence” and that it had removed listings for “Wrath” and “Natural Selection” shirts flagged by WIRED.

None of the other retailers immediately replied to a request for comment.

Katherine Keneally, ISD’s senior director of threat analysis and prevention, tells WIRED that these clothes are promoted by a cohort known as the True Crime Community, or TCC. Not to be confused with fans of podcasts and documentaries about unsolved or unusual crimes, the TCC “is a predominantly online fandom that is rooted in an obsession with mass killers, regardless of the mass killer’s motivation,” Keneally says.

The TCC’s fascination with the lore around specific killers can lead to dissection of details as minor as what they had for breakfast on the morning of their attack, Keneally says. But the statement shirts offer a convenient way to covertly endorse such massacres, with most people outside the in-group none the wiser.

“What we see from attackers is that they are often trying, in many ways, to mirror previous attackers, but putting their own sort of spin on it,” Keneally says. The shirts are used to “signal interest and glorification of the violence, and also can serve as a sign of someone's plans to engage in something, engage in some sort of attack,” she adds. The ISD’s report cites the cases of a 13-year-old arrested in South Africa last month after allegedly stabbing a 6-year-old while wearing a “Wrath” shirt and an 18-year-old accused of fatally shooting two people in a California library in June, who was apprehended wearing a “Natural Selection” tee.

The ISD analysis of the sale of shirts connected to past shootings is part of a planned series by the nonprofit covering various warning signs of future attacks. The idea is to give parents, teachers, and law enforcement more guidance in understanding when a young person may have intent to harm others, particularly because “within the TCC, we are seeing predominantly youth and minors being involved,” Keneally says.

Adolescents aren’t just engaging with this content in “the darker corners of the internet,” Keneally says. “It's happening very openly across mainstream and fringe platforms.” The prevalence of the clothing across an array of online shops is an indication of how the TCC’s extremist messaging flies under the radar.

Meanwhile, as ISD revealed in its research, younger affiliates of this diffuse collective are trading tips on how to disguise the meaning of paraphernalia tied to mass shooter fandom. One screenshot in the report shows a TikTok user asking their followers, “How do y’all convince your parents to buy the natural selection shirt,” with commenters advising that they pretend “Natural Selection” is the name of a movie, a video game, or a brand. One person replied: “My parents don’t know what tcc is so i can buy it without any problems.”

TCC enthusiasts have also made it clear in their product reviews of the T-shirts that they admire perpetrators of deadly gun violence. In their five-star feedback on a “Wrath” shirt, a customer wrote “we love dylan klebold!” Another attached an anime-style cartoon of Eric Harris in the “Natural Selection” shirt on their review of same, noting that they were hiding it from their mom but that it was “so cute” and “good quality too.” A third person ominously noted that they would use their “Wrath” shirt “to become a bit more popular at school.”

Keneally acknowledges that there’s a “fine line” to consider with clothing that may suggest an unhealthy fixation on infamous killers, since even the best-known examples are not offensive or illegal in themselves. Context also matters: The TCC has an affinity for the German industrial band KMFDM because they were a favorite band of the Columbine shooters, but a fan wearing a KMFDM shirt could have no idea of the connection.

While a teen’s purchase of the most flagrant TCC shirt doesn’t help them commit an attack or necessarily indicate the planning of real-world violence, Keneally says, adults should weigh it against other factors. “If they have access to weapons or maybe another student has heard them talk about committing violence, then it can help serve as a warning sign and provide an opportunity for intervention,” she says. And wearing the shirt at all communicates a likely need for mental health support.

As for the retailers that facilitate the sale of this apparel, Keneally believes that if they adequately enforced their established safety policies, much of it would be taken down. The clothes are something that youths seek out to be “a part of” the True Crime Community and fuel this toxic subculture, she says. “It shouldn't be that easy to access.”