The number of confirmed and presumed E. coli cases across Oregon and Washington state has jumped to 37 from 22 — with the majority linked to Chipotle restaurants, health officials said Tuesday.
The Portland metro area is now at 12 cases, up from three announced Monday, while Washington has seen 25 cases, up from the 19 people who have suffered from food poisoning from the bacterial illness.
The cases in Oregon date to at least Oct. 7, and the number could rise as people show symptoms following an incubation period that could stretch as long as 10 days, health officials said.
Chipotle said eight restaurants are involved in this outbreak in Washington and in metro Portland, although the company over the weekend closed 43 of its locations across both states as a precaution. They all remained shuttered Tuesday.
Related: Oregon Woman Became Severely Ill in Chipotle E. Coli Outbreak
Besides deep cleaning the restaurants, Chipotle said, it is retaining two consulting firms to help "assess and improve" its food safety standards.
"We are working with health department officials to determine the cause of this issue," Chipotle co-CEO Steve Ells said in a statement Tuesday. "We offer our deepest sympathies to those who have been affected by this situation."
Oregon State Health Officer Katrina Hedburg said officials continue to believe this latest outbreak strain is E. coli O26, and is from some type of fresh produce that is served at the popular Mexican chain — possibly lettuce, tomatoes, salsa or cilantro.
"It doesn't look likely it's a meat product and it's more likely it's a fresh-food item," Dr. Hedburg said at a news conference, adding, "The real key to this is finding out what food in the distribution process started this, and we do not have an answer to that."
E. coli, a foodborne bacteria, affects the intestines and can lead to cramping, severe diarrhea and vomiting — and can even cause death.
Of the 12 cases in Oregon, eight involve the person eating at a Chipotle, officials said. Three people had to be hospitalized for a few days, but all have been discharged.
In Washington, 23 of the 25 cases involved the person eating at the chain. Nine of them had to be hospitalized. At least 25 others are being observed for possible E. coli infection.
The ages of sickened people in Oregon and Washington over the past few weeks range from 5 to 61, and the young and the elderly are most at risk of becoming severely ill.
Hedburg said the source of the outbreak could be a farm or food distributor that specifically serves the Pacific Northwest since there have been no reports of cases elsewhere in the country.
"What distribution pattern is different in the Pacific Northwest than other places?" she asked.
Related: Chipotle Right to Close Restaurants After Food Outbreak, CDC Says
Among the apparent victims is Oregon woman Ashley Arellano, who told NBC News that she ate at a Portland-area Chipotle on Oct. 17 and became violently ill four days later.
"I was terrified that I was going to go to bed and not wake up," she said.
Bill Marler, a prominent food safety litigator in Seattle, said he commends Chipotle for acting quickly during this outbreak, but warned that finding out the exact cause won't be easy.
"It would be unlikely that they would have the same lot of cilantro or lettuce in the facility anymore at all," he told NBC News. "And at this point, people ate the evidence."
