Late Saturday morning, a young white man entered a Walmart in El Paso, Texas, and opened fire with an AK-47-style rifle, killing 20 people and injuring 26 more. Police apprehended the suspected shooter, a 21-year-old man from Allen, Texas. It didn’t take long for internet researchers to find a four-page manifesto posted less than an hour before the violence, a document that El Paso’s police chief appeared to later confirm was being examined by officials. The document had been uploaded to the notorious, unmoderated message board 8chan at 10:15 a.m. local time, and it included a request: “Do your part and spread this brothers!”

In the anti-immigrant screed, the author describes plans to perpetrate a mass shooting against the Latino people he believes are invading the United States, the weapons he will use to do it, and his white nationalist beliefs. Soon after it was first posted on 8chan, the manifesto could be found on 4chan, another message board with scant rules about what people can or cannot share. And not long after that, it was circulating as images on Twitter and Facebook and easily findable in a Google image search. Clearly someone got the author’s message.

The writer of the manifesto said he was inspired by the mosque shootings in Christchurch, New Zealand—before which the gunman, another young white man, posted a sprawling essay on 8chan as well as a link to a Facebook Live broadcast of the massacre and instructions to share. That video was removed about 45 minutes after the broadcast began, giving his audience ample time to copy the footage. It was posted to Facebook 1.5 million times in the first 24 hours after the shooting.

The involvement of 8chan is becoming a familiar detail in cases of white supremacist violence. The El Paso shooting appears to be the second one since the Christchurch massacre to draw from that killer’s playbook. About five weeks later, two hours before another young man murdered someone at a synagogue in Poway, California, the suspected shooter also posted a manifesto to 8chan. In it, he detailed his ideological rationale for mass murder and gave instructions to spread his words far and wide. Each of these shootings appeared to have been designed to go viral: A horrific act would catch the world’s attention, and a manifesto would deliver the hate-filled payload.

An anonymous, meme-filled internet backwater, 8chan has long been a place for white supremacists to indoctrinate others—mostly young white men—into bigoted ideologies. It’s difficult to put an exact time stamp on when 8chan and 4chan became prime hangouts for white nationalists and neo-Nazis, but the connections to other online venues of hate are clear.

Before 8chan and 4chan, there was Stormfront, a message board created by Ku Klux Klan Grand Wizard Don Black in 1996. For more than a decade, Stormfront was the most popular white supremacist website on the web. 4chan, founded in 2003, had become fashionable among the generally left-leaning online activists Anonymous, who were known for launching online trolling campaigns against those they politically disagreed with. There, the /pol/ message board (short for “politically incorrect”) attracted users who enjoyed trolling people with jokes about Nazis and racist tropes. Around 2012, Stormfront users reportedly noticed an opportunity to make 4channers “racially aware,” and that year, a “Redpill instruction pamphlet” to indoctrinate 4chan trolls began circulating on 4chan, according to a report in Rolling Stone. Andrew Anglin, who founded the influential neo-Nazi site the Daily Stormer, once said that 4chan is where he “got into Hitler.” Anglin later teamed up with Andrew “weev” Auernheimer, an infamous neo-Nazi troll who led vicious doxing campaigns against women, and the two helped to pioneer a meme-ified style of spreading hate that remains popular on 4chan and 8chan and across the fringe-right web today: make lots of jokes about the Holocaust and other genocides, desensitizing people to extreme hate-based violence.

If 4chan’s /pol/ could get pretty bad, 8chan became a destination for the very worst. The board was founded in the U.S. by Fredrick Brennan after 4chan’s owners decided to ban threads about the online harassment campaign Gamergate. While 4chan has a virtually nonexistent moderation policy, it does have rules against posting anyone’s personal information, like home addresses and phone numbers. 8chan doesn’t even have that. Whatever is too gruesome for 4chan finds a home on 8chan. That now includes enthusiasm for a white ethnostate. 8chan, now based in the Philippines, has “less restrictive moderation policies” than almost anywhere else on the internet, which has led it to become a “naturally more transgressional space” than anywhere else on the internet, according to Benjamin T. Decker, the founder of Memetica, a digital investigations consultancy.

Many people come to the politically incorrect boards of 4chan and 8chan from video game communities, where players looking to laugh at an abasing joke or chat about violent games without offending anyone can find friends. The forums are rife with in-jokes in which users routinely blur the lines between actually believing in Nazism and laughing about it. After the Poway shooter’s manifesto appeared, posters on 8chan wrote that the killer should try to “get the high score” and kill a larger number of people than the Chistchurch shooter did.

8chan isn’t only a place where people find community in their hate, but it is where they go when they want to announce mass murder to an online army that can help spread their message. Even when 8chan’s owners do feel compelled to step in—in a rarity, the suspected El Paso shooter’s original thread appears to have been removed—that army will have already gotten to work. The manifesto has been reposted again and again and again. The playbook worked.