Chipotle's latest E. coli cases may remain unsolved mystery

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Chipotle Mexican Grill said on Thursday it may never pinpoint what caused an E. coli outbreak last month linked to two restaurants in Kansas and Oklahoma, extending a mystery over dozens of illnesses tied to the burrito chain.

Chipotle no longer has any of the potentially contaminated ingredients that could be examined to determine the source of the infections, company spokesman Chris Arnold said. "It is doubtful that testing will ever be able to determine for sure what the cause of this was," he said.

SEE EARLIER: CDC investigating five new cases of E. coli linked to Chipotle

Company and health officials investigating the outbreak have also cited the same hurdles to tracing the cause of a larger E. coli outbreak which has sickened at least 50 Chipotle customers in nine states since October.

Chipotle has been under a microscope for months as customer illnesses stemming from E. coli and norovirus have battered sales and the company's stock price.

Identifying a specific cause for the latest E. coli cases "really can be a challenge with the limited information that may be available from the few cases that occurred," said Laurence Burnsed, an epidemiologist for the Oklahoma State Department of Health.

See photos of Chipotle stores forced to close because of E.coli

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Chipotle closings because of E.coli
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Chipotle's latest E. coli cases may remain unsolved mystery
Passers-by walk near a closed Chipotle restaurant on Monday, Dec. 7, 2015, in the Cleveland Circle neighborhood of Boston. Chipotle said late Monday that it closed the restaurant after several students at Boston College, including members of the menâs basketball team, reported âgastrointestinal symptomsâ after eating at the chain. The school said it was working with local health officials to determine the cause of the illness. (AP Photo/Steven Senne)
A sign is posted on the door to a Chipotle restaurant, Monday, Dec. 7, 2015, in the Cleveland Circle neighborhood of Boston. Chipotle said late Monday that it closed the restaurant after several students at Boston College, including members of the menâs basketball team, reported âgastrointestinal symptomsâ after eating at the chain. The school said it was working with local health officials to determine the cause of the illness. (AP Photo/Steven Senne)
A customer leaves a Chipotle restaurant in Seattle as the company started to reopen the outlets closed because of an E. coli outbreak in the Pacific Northwest, Wednesday, Nov. 11, 2015. (AP Photo/Donna Blankinship)
Customers leave a Chipotle restaurant with food in Portland, Ore., Wednesday, Nov. 11, 2015. Chipotle started reopening its restaurants in the Pacific Northwest on Wednesday after an E. coli outbreak sickened about 45 people, a high-profile example of foodborne illnesses that are more common than the public realizes, health experts say. Forty-three outposts of the Mexican food chain in Washington state and the Portland, Oregon, area were closed at the end of October because of the outbreak that hospitalized more than a dozen people. The first restaurants opened for lunch Wednesday. (AP Photo/Don Ryan)
A customer enjoys lunch at a Chipotle restaurant in Portland, Ore., Wednesday, Nov. 11, 2015. Chipotle started reopening its restaurants in the Pacific Northwest on Wednesday after an E. coli outbreak sickened about 45 people, a high-profile example of foodborne illnesses that are more common than the public realizes, health experts say. Forty-three outposts of the Mexican food chain in Washington state and the Portland, Oregon, area were closed at the end of October because of the outbreak that hospitalized more than a dozen people. The first restaurants opened for lunch Wednesday. (AP Photo/Don Ryan)
Rosanna Fleming enters a Chipotle restaurant for lunch as another customer enters a second door in the background in Portland, Ore., Wednesday, Nov. 11, 2015. Chipotle started reopening its restaurants in the Pacific Northwest on Wednesday after an E. coli outbreak sickened about 45 people, a high-profile example of foodborne illnesses that are more common than the public realizes, health experts say. Forty-three outposts of the Mexican food chain in Washington state and the Portland, Oregon, area were closed at the end of October because of the outbreak that hospitalized more than a dozen people. The first restaurants opened for lunch Wednesday. (AP Photo/Don Ryan)
Workers clean inside a still-closed Chipotle restaurant Monday, Nov. 9, 2015, in Seattle. Health officials in Washington and Oregon have said that more than three dozen people have gotten sick with E. coli in an outbreak linked to Chipotle restaurants in the two states. More than 40 Chipotle restaurants remain closed in Washington state and the Portland area while authorities search for the cause. (AP Photo/Elaine Thompson)
FILE - In this Nov. 9, 2015, file photo, pedestrians walk past a still-closed Chipotle restaurant in Seattle. An outbreak of E. coli that originated in the Pacific Northwest has spread south and east and has now infected people in six states. New cases have been reported in California, New York and Ohio, the Centers for Disease Control and Prevention said Friday, Nov. 20, 2015. (AP Photo/Elaine Thompson, File)
A worker sits at a counter inside a still-closed Chipotle restaurant Monday, Nov. 9, 2015, in Seattle. Health officials in Washington and Oregon have said that more than three dozen people have gotten sick with E. coli in an outbreak linked to Chipotle restaurants in the two states. More than 40 Chipotle restaurants remain closed in Washington state and the Portland area while authorities search for the cause. (AP Photo/Elaine Thompson)
Workers clean inside a still-closed Chipotle restaurant Monday, Nov. 9, 2015, in Seattle. Health officials in Washington and Oregon have said that more than three dozen people have gotten sick with E. coli in an outbreak linked to Chipotle restaurants in the two states. More than 40 Chipotle restaurants remain closed in Washington state and the Portland area while authorities search for the cause. (AP Photo/Elaine Thompson)
A closed Chipotle restaurant is shown, Tuesday, Nov. 3, 2015, in Federal Way, Wash. Chipotle closed 43 of its Pacific Northwest locations after the chain's third foodborne illness this year sickened about two dozen people. (AP Photo/Ted S. Warren)
A pedestrian, right, walks past a closed Chipotle restaurant Monday, Nov. 2, 2015, in Seattle. An E. coli outbreak linked to Chipotle restaurants in Washington state and Oregon has sickened nearly two dozen people in the third outbreak of foodborne illness at the popular chain this year. Cases of the bacterial illness were traced to six of the fast-casual Mexican food restaurants, but the company voluntarily closed down 43 of its locations in the two states as a precaution. (AP Photo/Elaine Thompson)
A sign posted on the door of a Chipotle restaurant in Portland, Ore. reads that the location is "temporarily closed due to a supply chain issue," on Monday, Nov. 2, 2015. An E. coli outbreak linked to Chipotle restaurants in Washington state and Oregon has sickened nearly two dozen people in the third outbreak of food borne illness at the popular chain this year. (AP Photo/Don Ryan)
FILE - In this Nov. 2, 2015, file photo, a pedestrian walks past a closed Chipotle restaurant in Seattle. Washington state health officials say they have found no source for the E. coli outbreak related to Chipotle and the chain's Pacific Northwest restaurants could reopen later this week. Washington state epidemiologist Dr. Scott Lindquist says all the tests of food from Chipotle stores in Washington and Oregon came back negative for E. coli. (AP Photo/Elaine Thompson, File)
A woman talks on the phone as she stands in the kitchen area of a closed Chipotle restaurant, Monday, Nov. 2, 2015, in Seattle. An E. coli outbreak linked to Chipotle restaurants in Washington state and Oregon has sickened nearly two dozen people in the third outbreak of foodborne illness at the popular chain this year. Cases of the bacterial illness were traced to six of the fast-casual Mexican food restaurants, but the company voluntarily closed down 43 of its locations in the two states as a precaution. (AP Photo/Elaine Thompson)
Signage hangs from a closed Chipotle restaurant in Portland, Ore., Monday, Nov. 2, 2015. Chipotle voluntarily closed down 43 of its locations in Washington and the Portland area as a precaution after an E. coli outbreak linked to six of its restaurants in the two states has sickened nearly two dozen people. (AP Photo/Don Ryan)
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Samples of multiple ingredients from a Chipotle near the University of Oklahoma campus in Norman are being tested at a state lab in Oklahoma City, Burnsed said. Three people who reported eating at the restaurant became sick with E. coli from Nov. 18-26, according to information from Chipotle and the U.S. Centers for Disease Control and Prevention (CDC).

Chipotle's most recent outbreak affected five people from three states who ate at restaurants in Kansas and Oklahoma in November, the CDC said. It is unclear whether the new cases are related to the larger outbreak that started in October, according to the agency.

Chipotle is investigating the cases and working with the CDC and U.S. Food and Drug Administration, Arnold said. The company has said it plans to roll out a new food safety plan that would "eliminate or mitigate risk to a level as near zero as possible."

But Marion Nestle, a professor in the department of nutrition at New York University, said "you can't make an easy prevention step because you don't know what it is that you're trying to prevent."

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