(Bloomberg) -- The U.S. Centers for Disease Control andPrevention has concluded its investigation into two E. colioutbreaks that sickened dozens of Chipotle Mexican Grill Inc.customers and thrust the chain into a crisis that wiped out more than $10 billion in marketvalue.

The CDC was unable to determine the source of contamination butsaid it was probably a common meal item or ingredient. Chipotleshares rose on the news, as investors bet that the worst was overfor the company after almost three months of negativeheadlines.

Chipotle’s reputation, built on the promise of fast food madewith fresh, locally sourced ingredients, has been battered inrecent months. The two E. coli outbreaks, which sickened at least60 customers in 14 states, raised questions about the chain’sprotocols for handling ingredients. Chipotle has apologized formaking customers sick and announced changes to it says will make itan industry leader in food safety.

Related: Chipotle's Seattle woes put fresh scrutiny onsafety procedures

Chipotle, which is scheduled to report earnings on Tuesday, saidlast month that sales plunged almost 15% in the fourth quarter, thefirst decline for the chain as a public company. The company alsodisclosed that it had been served with a grand jury subpoena in afederal grand jury investigation in California.

The criminal probe stems from a norovirus outbreak last summerat a restaurant in California, where more than 200 workers andcustomers were sickened. A Chipotle restaurant in Boston was shuttered forweeks in December after a norovirus outbreak there sickened morethan 140 students from nearby Boston College.

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