Green, whole head cabbage was the likely source of E. coli O111 illnesses traced to eating at Applebee’s restaurants in Minnesota this summer, state health officials said Friday.

Fifteen people were sickened by the produce, which was likely contaminated before it was distributed to restaurants, the department said. Health investigators interviewed 14 who were taken ill: 13 ate at nine Applebee’s restaurants in Minnesota; one ate at a Yard House restaurant.

Many of the cases involved people eating the oriental chicken salad at Applebee’s. This particular strain of E. coli O111 had not been seen in the United States previously, the department said.

Applebee’s pulled the item off its menus and returned it after tapping a different source for the ingredients.

Minnesota investigators traced the cabbage to a common supplier outside of Minnesota and continue to work with the U.S. Food and Drug Administration to investigate its source. “The FDA examination of the potentially involved farms is still ongoing. Single cases of illness that match the outbreak strain have occurred in three other states,” the department said.