Twenty-five Hormel Foods employees will be questioned as the state health department expands its investigation into a mysterious illness affecting as many as 13 Quality Pork Processors employees, a health official said this morning.

Dr. Ruth Lynfield, the state epidemiologist, said all of the employees work in the rendering portion of the plant directly below the “head table” in QPP’s section of the plant that has until now been the focus of the investigation.

“We are evaluating the people that work in that area because there is a likely case who had worked in that area,” Lynfield said. Lynfield said approximately 25 people work in that portion of the plant.

The news comes after a 13th employee in the plant developed symptoms consistent with an illness afflicting 12 QPP employees who worked at the “head table,” where the plant once used air compressors to extract brain tissue from hog skulls for food. The air compressor system has been viewed by officials as a likely culprit for the illness, as medical experts have said the workers could have inhaled small particles of brain matter, sparking an immune system reaction that causes the body to mistakenly attack its own nerve tissue.

The air compressor process has since been halted at QPP and a processing plant in Indiana, where two employees reported similar symptoms. The third plant in the United States to use the process, which the New York Times identified as a Hormel-owned plant in Nebraska, also stopped the process, though it reported no employees with similar symptoms.

Last week, the health department said it was expanding the investigation of the QPP section of the plant to thousands of former meatpackers going back a decade, when the air compressor system was first installed.

Health officials have maintained that food processed in the plant remains safe to eat.