(CNN) -- North Carolina public health officials revised Monday the number of confirmed and suspected cases of E. coli infection in an outbreak that appears to be linked to the state fair.
Nine cases have been confirmed and another 15 cases are under investigation, according to North Carolina Division of Public Health spokeswoman Julie Henry. That's two fewer cases than health officials initially released.
The revision comes as a result of new criteria applied to the outbreak, including lab evidence showing the same genetic fingerprint, whether patients have had symptoms since October 13 and whether patients attended the fair during the incubation period, Henry said.
While Henry noted Monday that the outbreak appears to be slowing, she added that health officials will be watching for new cases to emerge over the next week and a half.
The first illnesses were reported around October 15, Henry said last week. All 24 victims attended the state fair, which ran from October 13 through October 23 in Raleigh and drew nearly a million attendees. It remains unclear what at the fair may have caused the illness.
In an attempt to home in on the source of the outbreak, investigators are asking people who were sickened what they ate and what activities they participated in, she said. They will then compare the responses with those from fair attendees who did not get sick. But Henry acknowledged Friday that officials may never determine the source of the outbreak.
Symptoms of E. coli include abdominal cramps, fever, nausea, vomiting and watery or bloody diarrhea.
After the initial cases were reported, it did not take long for suspicion to be directed at the fair, since an E. coli outbreak in 2005 was traced to a petting zoo at the fair, Henry said.
Officials concerned about the possibility of more cases have alerted hospitals, physicians and county health departments to be on the lookout for patients with E. coli symptoms.
The bacteria are found in the feces of animals such as cattle, sheep and goats. Some types of E. coli can sicken people who drink liquids or eat food that has come into contact with the bacteria or who themselves come into contact with infected animals, according to the Department of Health and Human Services.
The majority of U.S. outbreaks -- blamed for some 265,000 illnesses and 100 deaths in the United States each year -- are linked to raw or undercooked ground beef, unpasteurized milk, unpasteurized juice and leafy greens, HHS said on its website.
Brian Long, director of the North Carolina State Fair press office, said last week that fair officials are cooperating with public health officials as they try to determine the source of the outbreak. "Right now, there are still far more questions than answers, and we know the investigation will take time. But we are eager to help investigators any way we can," he told CNN.