The E. coli outbreaks at Chipotle Mexican Grill restaurants in the United States appear to be over, but investigators have been unable to trace the cause of the sickness, federal authorities said on Monday.

The Centers for Disease Control and Prevention said they had investigated two outbreaks that occurred last year: one in 11 states that sickened 55 people and another that sickened five people in three states. Twenty-two people were admitted to hospitals. There were no deaths.

The C.D.C. said in a statement that a common meal or ingredient served at the restaurants was a likely source of the outbreaks, but investigators were unable to specify the food or ingredient responsible for the contamination.

“Most ill people in these outbreaks ate many of the same food items at a Chipotle Mexican Grill restaurant,” the statement said. “When a restaurant serves foods with several ingredients that are mixed or cooked together and then used in multiple menu items, it can be more difficult for epidemiologic studies to identify the specific ingredient that is contaminated.”