The U.S. Centers for Disease Control and Prevention announced on Monday it is investigating five more cases of illness from E. coli in Kansas, North Dakota and Oklahoma linked to Chipotle Mexican Grill. The CDC says the strain of E. coli in this latest outbreak is different from the one that sickened 53 people in nine states earlier in the fall.

Chipotle’s stock price fell to a 52-week low Monday following the news before recovering slightly.

Even though the strains of bacteria are different, it’s not clear to officials whether the outbreak is separate from or a continuation of the nine-state outbreak that sickened dozens throughout the Pacific Northwest and Northeast from mid-October to early November. In the most recent cases, customers became sick between Nov. 18 and 26. Of the five people who fell ill, one was from Kansas, one was from North Dakota and three were from Oklahoma. They all ate at Chipotle in the week prior to their sickness.

During the early days of the first outbreak, Chipotle said it expected to see reports of additional E. coli cases because there is a lag in time between contracting E. coli and the onset of symptoms and some consumers may not immediately report a sickness. CEO Steve Ells has repeatedly assured customers that the company believes there is no E. coli currently in its restaurants.  

Since August, Chipotle has been mired in one food safety issue after another. In addition to the E. coli cases, the popular burrito chain has suffered two norovirus outbreaks and the discovery of salmonella in its tomatoes at 22 restaurants in the Midwest last summer. The company unveiled a new food safety plan in December in partnership with IEH Laboratories and Consulting Group in Seattle that includes repeated testing of freshly prepared ingredients, improved employee training and extra precautions during preparation such as blanching fresh greens in boiling water for a few seconds to kill bacteria.