WASHINGTON - The federal agency that monitors public health says the outbreak of E. coli illness linked to Chipotle restaurants that sickened 60 people appears to be over.

The Centers for Disease Control and Prevention said Monday the most recent illness reported to the agency started on Dec. 1.

The source is still unknown, however.

Chipotle Mexican Grill Inc. undertook an aggressive revamping of food preparation methods at its more than 1,900 locations. But the outbreak drove the company's stock down by 26 percent over the past three months and it warned of a potentially sizeable hit to profits last month.

Shares are rebounding sharply in midday trading.