Health investigators are trying to find out how three toddlers who attend the same day-care center in the Nebraska Panhandle contracted the E. coli virus, according to KETV7 in Omaha.

The three children range in age from 9 to 18 months and were being cared for in the Blues Clues Room at the Here Wee Grow center in Sidney. Two of the children remain hospitalized. The third child has left a hospital and is being cared for at home.

State investigators remain unsure of the source of the E. coli, which is normally a food-borne illness.

The classroom and the center’s food and water supplies have been tested. A direct tie of E. coli to the center has not yetbeen confirmed.