State investigators remain unsure about the exact source of the infection that sickened at least four children at the Here Wee Grow day-care center in Sidney, Nebraska.

Health officials are assuming that the infection was most likely caused by E. coli.

The classroom and the center’s food and water supplies have been tested, but a direct tie of E. coli to the center has not been confirmed. The center was still open Wednesday but one of its rooms remained closed for cleaning and sterilization.

At this time, health officials believe the source of the bacteria was outside the facility. The day-care staff and its board of directors are working closely with the Centers for Disease Control and Prevention, the Panhandle Public Health Department and the state epidemiologist to ensure the bacteria is contained and that the risk of further infection is eliminated.