Fredrick Brennan was getting ready for church at his home in the Philippines when the news of a mass shooting in El Paso arrived. His response was immediate and instinctive.

“Whenever I hear about a mass shooting, I say, ‘All right, we have to research if there’s an 8chan connection,’” he said about the online message board he started in 2013.

It didn’t take him long to find one.

Moments before the El Paso shooting on Saturday, a four-page message whose author identified himself as the gunman appeared on 8chan. The person who posted the message encouraged his “brothers” on the site to spread the contents far and wide.

In recent months, 8chan has become a go-to resource for violent extremists. At least three mass shootings this year — including the mosque killings in Christchurch, New Zealand, and the synagogue shooting in Poway, Calif. — have been announced in advance on the site, often accompanied by racist writings that seem engineered to go viral on the internet.