A preliminary investigation into the E. coli outbreak in McDonald’s Quarter Pounders that killed at least one person in the U.S. and sickened 49 others points to chopped onions served on hamburgers, according to the Food and Drug Administration. We do. However, neither the company nor public health officials have publicly provided information on where the onions were grown or whether they were also shipped to other restaurants.

According to McDonald’s, the onions were brought in from a single source – then chopped and packaged as raw vegetables in individual bags and shipped to the restaurant. “It’s a process that processes raw onions in a facility and then ships them to McDonald’s,” the spokesperson told CBS News.

Diced onions and quarter pound patties were removed from the menu

The restaurant also said it is looking for a new regional supplier of fresh onions. McDonald’s has also removed chopped onion and quarter pound beef patties – both used for the Quarter Pounder burger – from its menus in affected areas.

McDonald’s said it had worked closely with federal food safety regulators since late last week when it was alerted about the possible outbreak. The company said efforts to identify the contamination source have been complicated by the scope of the problem and the popularity of its products.

There are more than 14,0000 McDonald’s stores across the country serving 1 million Quarter Pounders every two weeks in the affected area.

Outbreak of powerful E. coli strain

According to officials, the strain of E. coli in the outbreak — known as O157:H7 — produces a powerful toxin that damages the lining of the small intestine, if onions were confirmed as the source of infection. So it would be the first time that the strain has been implicated in a raw onion outbreak, the Centers for Disease Control and Prevention said.

At least 49 people have become ill from the deadly infection linked to the outbreak, which causes extreme nausea and vomiting, diarrhea and fever. One person – an older adult, has died and ten others have been hospitalized, including a child who developed a kidney disease called hemolytic uremic syndrome.

Most of the sick people are from Colorado

Most of those who have fallen ill — 26 in Colorado — are mostly in Mesa County, according to news reports. With this, nine cases were reported in Nebraska. Other patients have been reported in Iowa, Kansas, Missouri, Montana, Oregon, Utah, Wisconsin and Wyoming.

The first case involving the Quarter Pounder occurred in Colorado late last month. State health officials alerted the CDC on October 10 about an unusual increase in E. coli cases.

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