While white-steepled churches and President Barack Obama prepared to comfort a grieving town Sunday, federal agents planned to fan out to dozens of gun stores and shooting ranges across Connecticut, chasing leads they hoped would cast light on the life of school shooter Adam Lanza. Among the questions to be answered: Why did his mother, a well-to-do suburban divorcée, keep a cache of high-power weapons in the house? What experience did Lanza have with those guns? And, above all, what set him on a path to shoot and kill 20 children, along with the adults who tried to stop him?...