Although health department investigators have not been able to pinpoint the cause of a Salmonella outbreak in St. Charles, Illinois, they say evidence suggests it was the salad at Portillo’s restaurant.

Fifteen people have now been confirmed infected with the outbreak strain of Salmonella, according to a news release Monday from the Kane County Health Department.  Eleven of the case patients reported eating at Portillo’s restaurant and seven of those reported eating a salad.

It is not known how the salad might have become contaminated. Food samples collected from the restaurant all tested negative.

Two Portillo’s employees tested positive for Salmonella Typhimurium, but investigators identified them as likely victims of the outbreak, not the source. So far, 76 employees have been cleared to return to work after twice testing negative in tests conducted 48 hours apart.

The onset of the first illness was April 5 and onset of the last illness was April 30.  Three of the case patients became so ill they were hospitalized.

Kane County says it continues to work with the Illinois Department of Public Health and the DuPage, DeKalb and Chicago health departments in investigating the outbreak.  Eight of the victims were from Kane County, four from DuPage County, and one each from DeKalb County, Chicago and Minnesota.