(Olympia, WA)  --  Health officials are investigating E. coli-related illnesses believed to be linked to chicken salad purchased last month at Costco stores in Washington.  At least one case has been confirmed in King County, related to the consumption of chicken salad made with rotisserie chicken.  E. coli illnesses have also been linked to Costco chicken salad sold in Colorado, Montana, and Utah.  Washington's health department says it's working with the Centers for Disease Control to determine the exact source.