The two teenage suspects in this week’s deadly attack on a San Diego mosque appear to have written a 75-page document replete with neo-Nazi ideology, incel rage and racist meme culture drawn from the darkest corners of the internet. In an echo of the 2019 massacre at mosques in Christchurch, New Zealand, the suspects appeared to have worn body cameras that livestreamed their assault, video of which has circulated online. The suspects — identified by authorities as Caleb Vazquez and Cain Clark, teenagers who are believed to have first met online — killed three people at the Islamic Center of...