Diners at Red Lobster and Olive Garden restaurants in Iowa and Nebraska caught an intestinal illness tied to a rare type of parasite after eating salad mix that came from Mexico, the U.S. Food and Drug Administration said Friday.

The federal agency said its investigators traced the outbreak to four "illness clusters" of restaurants, which spokeswoman Theresa Eisenman later identified to CNN as Red Lobster and Olive Garden locations. The probe didn't find indications that any bags of salad mix with the parasite - known as cyclospora - were sold at U.S. grocery stores.