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DENVER — The CDC is investigating five new cases of a rare strain of E. coli linked to Chipotle Mexican Grill, the U.S. Centers for Disease Control and Prevention said Monday.

Three of the ill individuals ate at a location in Oklahoma and two ate at a Kansas location. Their symptoms began between November 18 and November 26 and all the individuals had consumed food from Chipotle within a week of becoming ill.

These five cases have a different DNA fingerprint than the rare strain of E. coli identified in the outbreak linked to the same restaurant chain that has caused 53 cases or illness in nine states since October. The CDC reported the latest of these cases on Monday, an individual from Pennsylvania who became ill in early November. The strain is shiga toxin-producing E. coli O26, or STEC O26.

The CDC would not say how likely or unlikely it is that all the cases with both DNA fingerprints are linked but said it is performing tests to make that determination and hopes to know in a few weeks. There have been multistate outbreaks of foodborne illness involving multiple DNA fingerprints in the past, according to Dr. Ian Williams, chief of the CDC’s Outbreak Response & Prevention Branch. He added, “These recent cases occurred several weeks after the last cases in the larger outbreak and in a different geographic location. This may suggest they do not share a common source, but the investigation is ongoing.”

Chipotle is working with the CDC, FDA, and state health officials to investigate. No single food item has been identified as a source of illness.

“We have indicated before that we expected that we may see additional cases stemming from this, and CDC is now reporting some additional cases. Since this issue began, we have completed a comprehensive reassessment of our food safety programs with an eye to finding best practices for each of the ingredients we use. We are now in the process of implementing those programs…,” Chris Arnold, Chipotle communications director, said in an email to CNN, emphasizing that the CDC does not know whether the new cases are linked.

The restaurant chain is also increasing food safety training of its employees and making improvements to its ingredient testing. The goal, Arnold said, is “near zero” food safety risk to consumers. To help in the endeavor, the company is working with outside food safety experts, including Mansour Samadpour, CEO of IEH Laboratories and Consulting Group. “This program eliminates or mitigates risk to a level near zero and will establish Chipotle as the industry leader in this area,” Samadpour said in a press release earlier this month.

The more well known strain of E. coli is O157, which is more likely to cause severe cases of illness.

Symptoms, which include diarrhea and abdominal pain, usually begin two to eight days after a person has been exposed to the bacteria and resolve within a week. Some cases are severe and patients can develop hemolytic uremic syndrome, or HUS, which is a type of kidney failure. There have been no cases of HUS or deaths from this outbreak, although 20 of those who have become ill have been hospitalized.

Chipotle has had a rough time with outbreaks in recent months. Earlier this month, health officials in Boston shuttered a location after an outbreak of norovirus caused at least 140 cases of gastrointestinal illness. In September, health officials in Minnesota identified tomatoes as the cause of an outbreak of Salmonella linked to the chain.