The federal Centers for Disease Control and Prevention said two E. coli outbreaks tied to the Chipotle Mexican Grill Inc. appear to be over, rallying shares of the beleaguered company.
In an update on its website, the CDC said it was closing its investigation, though it was unable to determine a food or ingredient responsible for the contamination.
"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," the CDC said.
Chipotle spokesman Chris Arnold said in an email that the company has taken "significant steps" to improve the safety of its food.
"We are pleased that the CDC has concluded its investigation, and we have offered our full cooperation throughout," he said.
Its stock rallied following the CDC announcement, closing Monday up $19.67, or 4.3 percent, at $472.64.
Chipotle's stock first started to climb when the market opened after the Wall Street Journal reported Sunday night that the outbreak was expected to be declared over.
Chipotle's sales have slipped since outbreaks of E. coli were tied to its restaurants. The chain's December sales were down 30 percent, according to a recent filing with the Securities and Exchange Commission.
In November, the Denver-based burrito chain temporarily closed 43 restaurants in Washington and Oregon after 22 E. coli cases were linked to its eateries. Those restaurants have since reopened.
A month later, 141 Boston College students were reported to have contracted norovirus after eating at a Chipotle in Brighton, Mass.
That same month, Chipotle was served with a federal grand jury subpoena in connection with a criminal investigation being conducted by the U.S. attorney's office for the Central District of California in conjunction with the U.S. Food and Drug Administration's Office of Criminal Investigations.
The investigation is related to a norovirus outbreak in August at a Los Angeles-area Chipotle restaurant. Chipotle has said it will fully cooperate in the probe.
The chain plans to close all of its U.S. stores for part of the day on Feb. 8 in order to hold a meeting with employees across the nation. The meeting is designed to address changes in food-safety procedures, as well as to allow employees to ask questions about the situation.