The top federal prosecutor in southern Illinois is stepping down next month to join a national law firm based in the St. Louis area. Stephen Wigginton's planned departure as U.S. attorney for the 38-county southern Illinois region was announced Tuesday. It takes effect on Dec. 11. Sworn in as U.S. attorney in August 2010, Wigginton will join the Armstrong Teasdale law firm's headquarters in Clayton, Missouri, just west of St. Louis. The firm says he will focus on class-action lawsuits, complex litigation involving corporate malfeasance, and whistleblower lawsuits.