In rambling writings full of vitriol against a wide range of people, the teenagers who attacked the Islamic Center of San Diego this week, killing three men and themselves, left little doubt about the models for their violence.Chief among them: the shooter who killed 51 people at two mosques in Christchurch, New Zealand, in 2019.Researchers who study extremism have long noted the resonance of the Christchurch attack among far-right assailants, attributing it to the extent of the violence, the document the killer posted concerning his views and actions, and — especially — his decision to livestream the massacre. Among those...