It began with a phone call on Halloween to report the sound of gunshots. When police arrived at a Margate home, a woman told them her brother often shot his guns at the house, according to court records, and that she sometimes feared him. That day, officers would enter his room and happen upon a disturbing collection of over 40 items: drawings of local schools and parks labeled with the N-word; a list of synagogues, Jewish businesses and the name of Congressman Jared Moskowitz; and a stockpile of six guns, several rounds of ammunition, smoke grenades and a Ghillie suit. John Lapinski Jr., 41, had amassed the arsenal despite multiple laws that ought to have prevented a man with both a felony record and a domestic violence injunction from purchasing guns, according to prosecutors. The circumstances of his arrest highlight the increasing precariousness of preventing acts of mass violence or domestic terrorism, which often seem to rely on chance encounters or tips from acquaintances and loved ones.