Investigators from the Virginia Department of Labor and Industry have not determined what caused more than 50 people to become ill Tuesday at the Virginia Produce plant in Hillsville. Air quality tests conducted Tuesday and overnight into Wednesday were clear, revealing no origin of what made the workers sick, Carroll County Sheriff J.B. Gardner said in a news release. Officials from the U.S. Department of Agriculture met with the plant’s owners Wednesday morning and determined that the produce inside the plant was safe for shipment, Gardner said.