The latest mass killing has renewed multiple controversies about mobilizing science and medicine to prevent violent crime. With a big push from Governor Cuomo, the New York State Legislature passed a law this week requiring psychotherapists to report any client thought “likely to engage” in violent behavior. The police would then confiscate any weapons that person might have. That has produced a backlash from some practitioners. President Obama and Congress are looking for ways that mental health agencies to share information so law enforcement can take action. Is the next step preventive detention for people who’ve never committed crimes? Is prediction possible—or is there no choice but to wait for violence to happen?