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.
The Delaware General Health District said Thursday that 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.
Chipotle CEO Brian Niccol said the retraining will start next week and the daily routine will be augmented. The company has been working to recover from food scares in late 2015 that sent sales plunging.
Shares of Chipotle were down 3 percent Thursday afternoon.