It has been nearly four years since 20 children and six educators were gunned down at Sandy Hook Elementary School in Newtown, Connecticut. Over the past three years, filmmakers have followed the town and its families as they tried to understand and recover from the massacre. Their new documentary, "Newtown," opened Wednesday in theaters across the country. Newtown, with a population of about 28,000, had recorded only one homicide in the 10 years before December 14, 2012. That...