SEATTLE (AP) — Chipotle’s industry-leading commitment to tracking its ingredients from farm to table is being put to the test by an E. coli outbreak that has sickened at least 37 people as of Tuesday, nearly all of whom ate recently at one of the chain’s restaurants in Washington state or Oregon.

Scientists also said Tuesday that they identified the specific microorganism responsible, which they believe was carried on fresh produce such as lettuce, tomatoes, cilantro or onions, or possibly even spices.

The chain of casual Mexican restaurants voluntarily closed 43 locations in the two states after health officials alerted the company to a growing number of E. coli cases involving people who shared one common experience: a meal at Chipotle during the last two weeks.

The numbers grew Tuesday from three to 12 probable cases in Portland area and from 19 to 25 probable cases in five counties near Seattle.

Dr. Scott Lindquist, Washington’s state epidemiologist, said the specific microorganism responsible for the outbreak is Shiga toxin-producing E. coli O26.

Now officials are doing more tests, on samples of human waste and on any E. coli found in food samples, looking for exact DNA matches. That, in turn, should indicate which ingredient carried it into the meals of diners, Lindquist said.

Food from the restaurants linked to the outbreak is being tested, and Lindquist said they may know by Wednesday which produce, if any, tests positive for the same bacteria.

Authorities have already asked Chipotle to turn over information about its food suppliers.

“We’re really relying on working closely with Chipotle,” said Dr. Katrina Hedberg, Oregon’s state epidemiologist.

Chipotle Mexican Grill Chairman Steve Ells said in a statement Tuesday that the company immediately closed the restaurants “out of an abundance of caution, even though only eight restaurants have drawn concern.”

The company also says it is doing its own tests in restaurants and distribution centers. It is fully sanitizing the restaurants, replacing all the food and testing batches of ingredients in its supply chain as it helps investigators, the statement said.

Multi-state outbreaks have increased sharply in recent years, the Centers for Disease Control and Prevention reported Tuesday. On average, two dozen occurred from 2010 to 2014, up from six a year from 1973 to 2010. That’s partly due to better detection, but food industry consolidation has meant companies ship to wider networks of grocery stores and restaurants now, so tainted products can spread more widely as well.

In this case, identifying the supplier of any contaminated produce should be easier because Chipotle uses traceability software, made by the Durham, North Carolina-based FoodLogiQ company, and has promised its consumers that it can trace every box of fresh ingredients from farm to table, in real time.

“Ideally, any of these trace-back systems should help,” said Jaydee Hanson, a senior policy analyst at the Center for Food Safety, a Washington, D.C.-based nonprofit that promotes food safety and sustainable agriculture.

Health officials believe knowing the distribution of food along Chipotle’s supply chain will give them useful information, said Dr. Jeff Duchin, health officer for Seattle and King County Public Health.

“We’re really hoping that the trace-back will give us some good clues about what the cause could be,” Duchin said.

He acknowledged, however, that this will be a difficult investigation because so many of the same ingredients go into multiple dishes at Chipotle.

In 2008, health officials initially suspected raw tomatoes to be the source of a salmonella outbreak that sickened people around the country, Hanson said. Without adequate ways of tracing tomatoes or other contaminated produce to their origins, everybody stopped buying tomatoes, everywhere.

In this case, Chipotle has been able to have a much more targeted response.

“Being able to quickly find where the problem is, is why our organization and others have been arguing for good traceability, from farm to fork,” Hanson said. “Chipotle is to be commended for trying to put in place a system to trace its supply, and hopefully it works this time.”

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AP Medical Writer Mike Stobbe in New York and reporters Gene Johnson in Seattle and Jonathan J. Cooper in Portland, Oregon, contributed to this story.