One person has died and 49 people have become ill following an E. coli outbreak linked to McDonald’s Quarter Pounder hamburgers, the Centers for Disease Control and Prevention said on Tuesday.

Most cases have been reported in Colorado and Nebraska. Initial investigations have suggested that the slivered onions served on Quarter Pounders are a “likely source of contamination,” according to the C.D.C., which cited the Food and Drug Administration.

The onions are “primarily used on Quarter Pounder hamburgers and not other menu items.”

McDonald’s said it would stop using the onions, and has halted sales of Quarter Pounders at restaurants in Colorado, Kansas, Utah, Wyoming, Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico and Oklahoma.

Federal regulators are still investigating whether MacDonald’s hamburger patties could also be a source of the illness.

In a statement posted to its website, McDonald’s emphasized that other items on its menu, including cheeseburgers and hamburgers, are unaffected. “Serving customers safely in every single restaurant, each and every day, is our top priority and something we’ll never compromise on,” the statement said.

Bill Marler, a personal-injury lawyer who focuses on food safety, said onions are notoriously difficult to grow cleanly and have been linked to other food-borne illnesses, like salmonella.

If onions really are to blame, he said, health officials will want to know whether the supplier exclusively provided them to McDonald’s or if they were shipped elsewhere.