State health officials are trying to find the source of an E. coli outbreak on Oahu that has sickened nine people and hospitalized four.

Since Feb. 19, eight local residents and one visitor have been affected by the same strain of E. coli. Six were children, the state Department of Health said Wednesday.

“We are concerned,” said Michele Nakata, program manager of the department’s Disease Investigation Branch. “Of the nine people, there were three that were diagnosed with hemolytic uremic syndrome or HUS. That is a serious complication.”

The strain, known as O157:H7, can cause severe abdominal cramps and diarrhea and may progress to bloody diarrhea and HUS, the Health Department said. HUS is a life-threatening symptom associated with kidney failure.

The nine who were ill have recovered or are improving.