The Food & Drug Administration has joined health officials in Washington and Oregon investigating an outbreak of E. coli 026 linked to food served at Chipotle Mexican Grill restaurants as the company embarks on its own environmental and food testing to pinpoint a cause.

As of Nov. 3, of the 25 people who became ill in Washington, 23 said they had eaten at the popular restaurant chain before getting sick. The Oregon Health Authority is reporting 12 illnesses associated with Chipotle restaurants in the Portland area, up from three cases first reported on Oct. 31.

“While the outbreak appears to be linked to food served at Chipotle restaurants, the food or other source of contamination hasn’t yet been determined and remains under investigation,” the Washington State Department of Health reported on Oct. 31.

In the meantime, Chipotle has voluntarily closed 43 restaurants in Washington and Oregon until further information on the cause of the outbreak is available. At least 14 restaurants were shuttered in the Portland area.

Chipotle said it closed 43 restaurants, even though only eight restaurants have drawn concern, and has retained two food-safety consulting firms to help the company assess its food-safety program.

The company said Nov. 3 it was conducting “deep cleaning” and sanitation of its restaurants in the area, along with environmental testing and food testing in the restaurants and distribution centers.

The restaurant chain said it is replacing all food items in the restaurants that were closed and batch testing some ingredients before resupplying.

“The safety of our customers and integrity of our food supply has always been our highest priority,” said Steve Ells, chairman and co-chief executive officer of Chipotle. “We work with a number of very fresh ingredients in order to serve our customers the highest-quality, best-tasting food we can. If there are opportunities to do better, we will push ourselves to find them and enhance our already high standards for food safety,” he said.

Consumers should not assume a company that focuses on local and fresh ingredients is immune to food-safety issues, said Bill Marler of the Seattle law firm Marler Clark. “People shouldn’t have a false sense of security that local means safer,” he said.

Marler, who represents foodborne disease victims, dished advice to the fast-food firm.

“If I was the Chipotle CEO, I would be having my food-safety team in my office yesterday asking, ‘What happened and what do we need to do to make sure this doesn’t happen again,” Marler said. “I guarantee the cost of any change they need to make to make their food supply safer pales in comparison to the losses they have had in just last 24 hours on their stock price.”

In the meantime, Washington and Oregon health officials are asking people who ate at the restaurant chain during October and became ill to see their health care providers and mention the ongoing outbreak.