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A preliminary investigation by the U.S. Food and Drug Administration suggested fresh slivered onions that are served raw on Quarter Pounder hamburgers were a likely source of . McDonald's also serves raw, slivered onions on one of its breakfast sandwiches, but that sandwich isn't available at the impacted stores. Other burgers, like , use diced, cooked onions.

McDonald's said it was searching for a new regional supplier for fresh onions. In the meantime, Quarter Pounders were removed from menus in Colorado, Kansas, Utah, Wyoming, and portions of Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico, and Oklahoma.

Adriean Madden pulled up outside a McDonald’s outside Denver on Wednesday for his usual afternoon snack but then decided against it. He said he was unsure how E. coli spreads or contaminates other foods, and he thinks McDonald’s should be more forthcoming.

“This affects my decision with coming to McDonald’s in the future,” Madden said. “I feel like the information isn’t as widely spread. I didn’t see any notices on the door, and then I saw vehicles going through the drive through just as if nothing was going on.”

Colorado has had the most reported cases of any state so far, and it's where the one death involving an older adult occurred.

McDonald’s said it has worked closely with federal food safety regulators since late last week, when it was alerted to the potential outbreak. The company said the scope of the problem and the have complicated efforts to identify the contamination source.

McDonald's has more than 14,000 U.S. stores and serves 1 million Quarter Pounders every two weeks in the affected 12-state area.

McDonald's is known for its stringent guidelines and protocols, said Chris Gaulke, a professor of food and beverage management at Cornell University’s Nolan School of Hotel Administration. The company said Wednesday that the supplier regularly tested its onions for E. coli, for example.

“Given the volume of food that they go through, how infrequently this happens to McDonald's is a testament to the effort that they take,” Gaulke said.

But some experts questioned why McDonald's simply stopped selling one sandwich and didn't close restaurants for further investigation.

“Good practice would have been to close all the restaurants," said Bill Marler, a Seattle lawyer who has sued companies over food poisoning outbreaks. “Until we know definitively what the product was that made people sick, consumers should be aware."

Marler said cross-contamination remains a potential possibility at the affected restaurants until they are thoroughly cleaned.

Asked why it didn't close any restaurants, McDonald’s said nothing in the government’s investigation indicated there were issues with its food preparation practices. In an interview on the “Today” show Wednesday, McDonald's U.S. President Joe Erlinger also said it's likely that whatever product was contaminated has already passed through the company's supply chain.

The Centers for Disease Control and Prevention reported the outbreak late Tuesday. It said infections were reported between Sept. 27 and Oct. 11 in Colorado, Iowa, Kansas, Missouri, Montana, Nebraska, Oregon, Utah, Wisconsin and Wyoming.

State and local public health officials were interviewing people about the foods they ate in the week before they got sick. Of the 18 people interviewed as of Tuesday, all reported eating at McDonald’s, and 16 people reported eating a beef hamburger. Twelve reported eating a Quarter Pounder.

McDonald's said it's unlikely in the Quarter Pounder was the source, since it comes from multiple suppliers and is cooked at a high enough temperature to kill E. coli.

McDonald's said its initial findings suggest that some of the reported illnesses were linked to onions from a single supplier, which the company didn't name. McDonald's said the onions are cleaned and sliced by the supplier and then packaged for use on individual Quarter Pounders.

The incubation period for E. coli is only a couple of days, so illness would be quickly apparent to anyone affected, said Donald Schaffner, a food safety expert at Rutgers University. “If you ate these burgers in September and now it’s the middle of October and you didn’t get sick, you’re probably OK,” he said.

are harbored in the guts of animals and found in the environment. Infections can cause severe illness, including fever, stomach cramps and bloody diarrhea. People who develop symptoms of E. coli poisoning should seek health care immediately and tell the provider what they ate.

The type of bacteria implicated in the McDonald's case causes about 74,000 infections in the U.S. annually, leading to more than 2,000 hospitalizations and 61 deaths each year, according to the CDC. In general, E. coli infections were lower in 2023 than in recent years and cases of severe kidney injury caused by the bacteria remained stable, according to latest federal data.

Outbreaks at restaurant chains are rare, but .

In 2020, agreed to pay a record $25 million fine to resolve criminal charges that it served tainted food that sickened more than 1,100 people with E. coli between 2015 and 2018. In that case, poor safety practices, such as not keeping food at proper temperatures to prevent pathogen growth, were to blame.

In 2006, Taco Bell ordered the removal of green onions from its restaurants nationwide after samples taken by investigators appeared to contain a harsh strain of E. coli. The outbreak sickened at least 71 people.

“The worst thing you can have at a restaurant is a food safety problem. It’s the equivalent of an airline losing the plane,” said Aaron Allen, a restaurant consultant and founder of Aaron Allen and Associates.

But Allen said McDonald's has enough experience and safety protocols in place that it won't suffer long-term damage from the outbreak.

“No one would be better equipped to mitigate and respond to this than McDonald's,” he said.

McDonald's shares fell 5% Wednesday.

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AP Health Writer JoNel Aleccia in Temecula, California, and AP Video Journalist Thomas Peipert in Edgewater, Colorado, contributed to this story.