Mr. Brennan, a former Brooklynite who cut ties with the site in December, said he believed 8chan’s administrators were too slow to remove the post last week from Christchurch, New Zealand, shooter Brenton Tarrant and posts on the site’s message boards that incite violence. Their reluctance to do so, along with the proliferation of posts on 8chan praising Mr. Tarrant’s actions, have persuaded Mr. Brennan that the toxic, white-supremacist culture that lives on parts of the site could someday be linked to another mass shooting.

“It was very difficult in the days that followed to know that I had created that site,” he said in an interview from the Philippines, where he has lived since 2014. He added: “It wouldn’t surprise me if this happens again.”

Mr. Brennan for years tended to one of the internet’s darkest corners, moderating the site as criticism mounted for its persistently racist content and tolerance for user behavior that was long ago banned by mainstream platforms. The site has also been criticized for users posting child pornography and coordinating aggressive harassment campaigns against critics. Mr. Brennan was an administrator on the site until 2016 and worked for the owners of 8chan until December.