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.

“If these onions went to other places, you could start to see this thing expand pretty quickly,” Mr. Marler said.

The C.D.C. called the situation “a fast-moving outbreak investigation.” Most people fell ill with the bacterial infection between late September and mid-October, and all of those interviewed by health officials said they had recently eaten at McDonald’s.

Health investigators are also trying to determine whether any contaminated ingredients had been sold by other retailers or grocery stores.

The outbreak has hospitalized 10 people in the Mountain States, including a child who has a complicating illness, according to the C.D.C. One Colorado resident has died.

Symptoms of E. coli infection typically start four days after consuming a contaminated food and include diarrhea and severe stomach cramps. Medical help is required if those symptoms last longer than two days, or if the patient experiences a fever higher than 102 degrees or dehydration.

In severe cases, E. coli may trigger kidney problems, which can be life-threatening.

McDonald’s has been unusually in the news this week. On Sunday, former President Donald Trump staged an event at a closed McDonald’s outside Philadelphia, working a fryer and serving fast food to a selected audience through a drive-through window.

The hope was to cast doubt on Vice President Kamala Harris’s claim that a summer college job she had was at a McDonald’s. That recollection has been corroborated by a family friend.