(CNN) -- Oklahoma health officials said Friday they are searching for the source of a rare form of E. coli that has killed one person and sickened 116 others in the northeastern part of the state.

The subtype of bacteria -- called E. coli 0111 -- is "not normally found in this form of outbreak," said Leslea Bennett-Webb, director of communication for the Oklahoma State Department of Health.

More than 50 people have been hospitalized and nine people -- six of them children -- have been placed on dialysis, she said.

She said the federal Centers for Disease Control and Prevention in Atlanta, Georgia, helped state officials determine the subtype, but said the cause of the outbreak remains unknown.

"The focus has been narrowed to the Country Cottage Restaurant located in Locust Grove," she said, noting that most of the people who became ill ate there between August 15 and August 23.

Tests carried out on water from a well on restaurant property indicate the presence of bacteria, but "we have not been able to confirm what kind of bacteria," said Skylar McElhaney, a spokeswoman for the Oklahoma Department of Environmental Quality.

The Oklahoma Department of Health will analyze them and compare them with samples taken from victims, she said. "We can't say for sure that it is tied to the water in any way, but we also cannot rule it out," she said.

Symptoms of infection with the bacteria can include severe diarrhea, bloody diarrhea, vomiting and severe abdominal cramping, said Larry Weatherford of the Oklahoma State Department of Health.

Management at the restaurant, which has closed during the investigation, was working closely with health officials, he added.

Meanwhile, the outbreak appears to be abating. "While we believe we are seeing a downward curve in the number of people who have become ill, we still have many challenges with some patients who remain hospitalized," said State Epidemiologist Dr. Kristy Bradley. "We continue to ask the public to be extra diligent in their hand washing and food preparation to minimize the possibility of additional persons becoming ill."

The CDC estimates there may be about 70,000 E. coli infections each year in the United States.

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