NEW YORK – The Centers for Disease Control and Prevention says it is investigating a more recent batch of E. coli cases linked to Chipotle, and that it does not know yet if they are linked to a larger outbreak that began in October.

So far, the agency said Monday five people have been reported sick in the new outbreak, with illnesses starting between Nov. 18 and Nov. 26. They include one person in Kansas, one in North Dakota and three in Oklahoma. All five individuals said they ate at a Chipotle the week before they got sick. For the larger outbreak, 53 people had been reported sick in nine states, with 46 of them saying they ate at a Chipotle. The most recent illness linked to Chipotle among those cases started Nov. 10. Each year, about 48 million people get sick from foodborne illnesses, according to the CDC. Following the outbreak, Chipotle Mexican Grill Inc. has said it is tightening standards to ensure food safety. Spokesman Chris Arnold said in an email Monday that the chain is in the process of implementing its new programs, which include increased testing of ingredients and food safety training for workers.

“We have indicated before that we expected that we may see additional cases stemming from this, and CDC is now reporting some additional cases,” Arnold said in the email.

The CDC has not identified the ingredient that sickened people in the larger outbreak, which has already scared away customers and caused Chipotle’s sales to plummet 16 percent in November. Chipotle co-CEO Steve Ells has told The Associated Press he doesn’t think the company will ever know for sure the exact ingredient that was responsible, but that he believes it was bacteria in fresh food like tomatoes or cilantro.

In its annual report, Chipotle has noted it may be at a higher risk for outbreaks of food-borne illnesses because of its “fresh produce and meats rather than frozen, and our reliance on employees cooking with traditional methods rather than automation.” Those points of differentiation have long been marketing strengths for Chipotle, which has sought to distinguish itself as being of higher quality than traditional fast-food chains.

As sales faltering in recent weeks, Chipotle last week took out full-page ads in 61 newspapers around the country apologizing for the illnesses and reassuring customers of its commitment to tightening its food safety standards.

Earlier this month, Chipotle also closed a restaurant in Boston after dozens of students at Boston College, including members of the men’s basketball team, reported gastrointestinal symptoms. Those cases were linked to norovirus, and Chipotle has said they were unrelated to the E. coli outbreak.

Shares of the company dropped 3.5 percent, or $19.07, to close at $522.01. Earlier in trading, the stock hit a 52-week low of $508.10.

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