Half a century after the mysterious abduction and killing of a 7-year-old Illinois girl, authorities found a suspect — a former neighbor of the child — secured his conviction and put an end to a small town’s agony.Jack McCullough, by that time a 73-year-old retired policeman, was sentenced to life in prison in 2012.But according to the findings of a new public prosecutor, the case should stand as a warning on the perils of reopening a decades-old investigation.A six-month review by DeKalb County State …