Early on a December morning, Robert Coogan pulled his red Chevy hatchback into the parking lot of the state prison in Saltillo, Mexico. It was frigid outside, the sun had not yet cleared the reddish mountains, and Coogan lingered, staring at the tall black letters on the prison’s high walls: “CERESO” — Centro de Reinserción Social, the place where criminals are supposed to be reformed. Coogan, who has served as chaplain at the prison for a decade, slowly pulled himself from the warm car. In dark jeans, brown boots and a thick gray sweater, he looked more like a factory foreman than a Brooklyn-born priest. He wore no clerical collar, just a necklace of pendants with images of the Virgin Mary and Christ on the cross.