Last July 4, a group of 11 protesters, among them a middle school teacher and a UPS worker, held what they called a “noise demonstration” outside the Prairieland Immigration and Customs Enforcement Detention Center in the town of Alvarado, Texas, about 30 miles south of Fort Worth. Some spray-painted epithets on cars; others, it being nighttime on the Fourth of July, set off fireworks. There was no real violence at first. But then, an Alvarado police officer, Thomas Gross, arrived on the scene