Chipotle restaurant workers fill orders for customers on April 27, 2015 in Miami, Florida. (Joe Raedle/Getty Images)

The E. coli outbreak linked to Chipotle restaurants in more than a dozen states appears to be over, the Centers for Disease Control and Prevention announced Monday.

At least 60 people were sickened by one of two E. coli strains. The most recent case of illness was reported Dec. 1, 2015.

While the outbreak came from "a common meal item or ingredient" served in Chipotle, its exact source remains unknown, according to the CDC. State and federal regulators who reviewed the company's distribution records couldn't identify a single item to explain the outbreak -- and neither did testing food from multiple restaurants.

"Most ill people in these outbreaks ate many of the same food items at a Chipotle Mexican Grill restaurant," the CDC said in a release." 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."

When cases first emerged in 2015, the company shuttered dozens of restaurants  and pledged to double-down on food safety protocols.

Chipotle's stock -- which had fallen dramatically last year when cases were fist reported -- was up by 4.7 percent Monday following the CDC announcement, Reuters reported.

Certain strains of E. coli can cause fever, vomiting and bloody diarrhea. Children and the elderly are the most at risk for falling ill from the bacteria.