As of Wednesday, 38 people have been hospitalized. Although many strains of E.coli are found in the intestines of healthy people and animals and are harmless, certain types can cause severe abdominal cramps, bloody diarrhea and vomiting.

The agency said among the 62 people it interviewed, 52 were reported to have eaten at a Wendy's restaurant in the week before their illness started.

The burger chain was not immediately available for comment.

The new cases are an increase from last Friday when the health agency launched an investigation after 37 people across Michigan, Ohio, Indiana and Pennsylvania took ill.

Many of them had sandwiches with romaine lettuce at the burger chain a week before they fell ill, the agency had said.

The CDC, however, is not advising against eating at Wendy's restaurants or asking people to avoid romaine lettuce as it is yet to confirm the exact cause of the outbreak.

The burger chain had last week said that as a precautionary measure it was removing the vegetable from its sandwiches at restaurants in the region.

The agency said the true number of sick people is likely to be higher than reported and the outbreak may not be limited to the states with known illnesses.

(Reporting by Leroy Leo in Bengaluru; Editing by Arun Koyyur)