Mr. Brennan started the online message board 8chan in 2013, as a spinoff of 4chan, the better-known message board. In its early years, the site was known as an unmoderated free-for-all site populated by anonymous posters, where shocking and offensive humor reigned.

Now, 8chan is known as something else: a megaphone for mass shooters, and a recruiting platform for violent white nationalists. And Mr. Brennan, who stopped working with the site’s current owner last year, is calling for it to be taken offline before it leads to further violence.

“Shut the site down,” Mr. Brennan said in an interview on Sunday. “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 realize it.”…

The site remains nearly completely unmoderated, and its commitment to keeping up even the most violent speech has made it a venue for extremists to test out ideas, share violent literature and cheer on the perpetrators of mass killings. Users on 8chan frequently lionize mass shooters using jokey internet vernacular, referring to their body counts as “high scores” and creating memes praising the killers.