(CBS) -- Health officials in Ohio on Thursday faulted a toxin that occurs when food is left at unsafe temperatures for an illness that struck 647 customers of a Chipotle Mexican Grill restaurant in the state.
Chipotle said it will retrain workers as a result.
The Delaware General Health District said food samples from the store tested negative for clostridium perfringens, but stool samples came back positive for the toxin it forms. Traci Whittaker of the health district says five of six stool samples came back positive. The health district says no specific food has been identified as the source.
The company had closed the Powell, Ohio, store last month for cleaning when it became aware of reported illnesses. That store reopened the next day, and health officials were awaiting tests to determine the source.
"Chipotle has a zero-tolerance policy for any violations of our stringent food safety standards and we are committed to doing all we can to ensure it does not happen again, CEO Brian Niccol said in an emailed statement. The burrito chain will be "retraining all restaurant employees nationwide beginning next week on food safety and wellness protocols," he added.
The company has been working to recover from a slew of high-profile safety scaresthat battered its brand and stock price in 2015 and 2016.
Shares of Chipotle closed down 4.4 percent on Thursday.