Several incidents of mass violence in 2019 were preceded by manifestoes posted to deep Web social media sites by their perpetrators. These sites, most notably 4chan and 8chan, are buried in the deep Web, away from the neutralizing effects of broad public discourse. Many of the posts to these sites reference earlier extremist incidents, and indeed the incidents themselves mimic aspects of previous attacks. Building on previous research, this paper examines these deep Web social media sites. Through an analysis of traffic and posts, we confirm that these sites often act as a self-reinforcing community of users encouraging each other to violence, and we map a statistically significant rise in post volume; on these sites immediately following terrorist attacks.