Federal microbiologists and food safety investigators have descended on the Danville, Va., plant that makes Nestle’s refrigerated cookie dough, trying to crack a scientific mystery surrounding a national outbreak of illness from E. coli 0157, a deadly strain of bacteria, which has been linked to the product.

Health officials and food producers puzzled Saturday over how E. coli 0157, a bacterium that lives in the intestines of cattle, could have ended up in a product that seems so unlikely to contain it. “It’s a fascinating outbreak,” said Craig Hedberg, an expert on food-borne diseases at the University of Minnesota. “By just looking at package labeling, there is no reason you would expect an event like this to occur.”

The outbreak, which has sickened at least 65 people in 29 states, is the latest worry for consumers in the Washington area and across the country unnerved by a wave of food-borne illnesses, including botulism associated with canned chili and infections from salmonella linked to peanut products.

With cookie dough, like peanut butter, being a favorite of children, the latest outbreak is particularly alarming because the young and the elderly are more likely to develop severe complications if infected with E. coli 0157. More than two-thirds of the 65 victims are younger than 19, according to the CDC.