Two E.coli outbreaks linked to Chipotle Mexican Grill Inc (CMG.N) that affected more than 50 people across 14 states appear to be over, the U.S. Centers for Disease Control and Prevention (CDC) said on Monday.

The announcement assuaged some investors' concerns, sending the popular burrito chain's shares up 4 percent.

The regulator, however, was not able to identify the ingredient that was responsible for the contamination after a three-month probe. (1.usa.gov/1WXAYhb)

The CDC said 55 people were affected in the initial E.coli outbreak that spread across 11 states, adding that a smaller outbreak involving a rare and different E.coli strain affected five people in three states.

Chipotle has been grappling to overcome a series of food-borne illnesses linked to its chain since October that have driven away diners, hammered its high-flying stock and spawned both a federal criminal probe and a shareholder lawsuit.

The company was served with a subpoena last month in a federal criminal probe linked to a norovirus outbreak in California in August.

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Adding to investors' worries, Chipotle also forecast in January its first-ever drop in quarterly same-store sales. The company is scheduled to report fourth-quarter results on Tuesday.

The most recent E.coli illness reported to the CDC was on Dec. 1, the regulator said on Monday.

Testing of multiple food items collected from Chipotle restaurant locations did not show the presence of the bacteria, the CDC said.

Chipotle said last month it was confident that the steps it had undertaken to tighten food safety would stop future food poisoning outbreaks.

The Wall Street Journal first reported the CDC's findings on Sunday.

Chipotle's stock has fallen nearly 30 percent since the end of October when the first case of E.coli was reported.