The Centers for Disease Control issued a food safety alert on Tuesday about an ongoing investigation into possible contaminated food at McDonald’s restaurants.

An E. coli outbreak in 10 states has been linked to McDonald’s Quarter Pounder. So far, 49 cases have been identified, with 10 persons requiring hospitalization. One person died from the E. coli-induced illness. Most of the individuals sickened reported eating a Quarter Pounder hamburger at McDonald’s prior to becoming ill.

A majority of the reported illnesses occurred in Colorado and Nebraska, but cases have also been reported in Kansas, Missouri, Iowa, Wisconsin, Wyoming, Montana, Utah, and Oregon. No cases of E. coli-related illness have been reported thus far in Texas. Still, some McDonald’s restaurants in the local area have temporarily removed the Quarter Pounder from their menu offerings as a precaution.

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Investigators are working to determine if the beef patties or the slivered onions on Quarter Pounders are the likely source of the E. coli contamination. According to the U.S. Food & Drug Administration, diced onions and other types of beef patties used at McDonald’s have not been implicated in the outbreak.

Taylor Farms, a supplier for McDonald’s, announced a recall of four raw onion products out of an abundance of caution “due to potential E. coli contamination.” The supplier urged restaurants to stop using and destroy the affected products.

Symptoms of an E. coli infection can begin “anywhere from a few days after consuming contaminated food or up to nine days later. Symptoms include severe stomach cramps, diarrhea, nausea, and/or vomiting,” the FDA states on its website. “Some infections can cause severe bloody diarrhea and lead to life-threatening conditions, such as a type of kidney failure called hemolytic uremic syndrome (HUS), or the development of high blood pressure, chronic kidney disease, and neurologic problems.”

“Consumers who have eaten at McDonald’s and have symptoms of E. coli infection should contact their health care provider to report their symptoms and receive care immediately,” the FDA website states.