By Alexandra Sifferlin
November 20, 2015

A strain E.coli linked to Mexican food chain Chipotle has now been identified in six different states, the U.S. Centers for Disease Control and Prevention (CDC) reports.

The agency reported on Friday that 45 people have been infected with a strain of E. coli in California, Minnesota, New York, Ohio, Oregon and Washington. Among those sick, 43 reported that they ate at a Chipotle in the week before they got sick. So far, 16 have been hospitalized.

The CDC says the investigation is ongoing, and while evidence suggests a common meal item or ingredient served at the chain restaurant is the likely source of these cases, the precise link has not been determined yet. Chipotle said it will continue to work with federal officials to determine the cause, adding it had already taken “aggressive steps” to ensure its restaurants are as safe as possible.

“We offer our sincerest apologies to those who have been affected,” said Chipotle chairman and co-CEO Steve Ells. “We will leave no stone unturned to ensure the safety of our food.”

Public health experts are conducting whole genome sequencing to better understand the bacteria that is making people ill. “Consumers should contact a health care provider if they recently became ill with diarrheal symptoms after eating at a Chipotle Mexican Grill restaurant,” the CDC said in a statement.

In October Chipotle closed 43 of its restaurants in Oregon and Washington, but they have since re-opened, the Associated Press reports.

Contact us at editors@time.com.

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