An extreme message board is understood to have served as a platform for the violent ramblings of the suspect in the El Paso mass shooting.
The perpetrators of the Christchurch mosque shootings and an attack on a US synagogue also posted links to documents and live streams of their violent sprees on 8chan.
What is 8chan and why are extremists using it?
What is it?
Founded in 2013, 8chan is a message board dedicated to extreme free speech and anonymity. Users set up discussion areas or "boards" and moderate the site. Although there are hundreds of topic areas, the site is most notorious for its "/pol/" board, short for "politically incorrect".
The board consists of a constant barrage of virulently racist, anti-Semitic, and conspiracy theory-related content. Its reliance on users to moderate and pick topics led to it becoming a key organising platform for extremist campaigns.
8chan /pol/ was a crucial meeting ground for activists behind the anti-feminist campaign known as GamerGate, and more recently spawned QAnon - a conspiracy theory popular with some supporters of President Donald Trump.
The absolutist free speech ethos also attracted people who want to announce their violent plans. Prior to the El Paso attack, a short document appeared on /pol/ which appeared to outline the shooter's beliefs. It appeared along with a letter from a college that included the shooter's name.
Similarly, 8chan posts were made by suspects in the March shooting in Christchurch, New Zealand, and the synagogue shooting in Poway, California in April, prior to the attacks.
Who owns it?
8chan was set up by Fredrick Brennan, a programmer known by the nickname "Hotwheels" who has said in interviews that he was frustrated with a similar message board, 4chan, and wanted to set up an alternative.
Mr Brennan later turned the site over to Jim Watkins, a US army veteran. Both men live in the Philippines. Mr Brennan has since distanced himself from 8chan. Following the attack, he told the New York Times: "Shut the site down".
He told the paper: "It's not doing the world any good. It's a complete negative to everybody except the users that are there. And you know what? It's a negative to them, too. They just don't realise it."
Although 8chan does not appear in Google search results, internet ranking company Alexa says it is among the top 5,000 websites worldwide.
The BBC has contacted 8chan for comment.
How has it stayed online?
The company that handles 8chan's domain registration, Tucows, said that it has no plans to deregister the site.
Another key company is Cloudflare, which protects websites against online attacks. Cloudflare has been extremely reluctant to pull its services from any website, no matter its subject matter, and only did so in one high-profile case involving a neo-Nazi site.
After the El Paso attack, Cloudflare chief executive Matthew Prince said initially that his firm would continue to support 8chan. However, he reversed course on Sunday and wrote that the company would stop providing services because "8chan has repeatedly proven itself to be a cesspool of hate".
"Even if 8chan may not have violated the letter of the law in refusing to moderate their hate-filled community, they have created an environment that revels in violating its spirit.”
By early Monday GMT, the site had gone offline after tweeting: "There might be some downtime in the next 24-48 hours while we find a solution." Administrators say the site has enlisted another US-based security firm, BitMitigate.
Blog by Mike Wendling