McDonald’s said Wednesday that customers should feel confident ordering from its restaurants despite a deadly E. coli outbreak linked to its Quarter Pounder hamburgers. The outbreak has sickened at least 49 people in 10 states, and one person has died.

A preliminary investigation by the U.S. Food and Drug Administration suggests fresh onions that are served raw on Quarter Pounders were a likely source of contamination.

The company said Wednesday it is searching for a new regional supplier for fresh onions, and that Quarter Pounders have been removed from menus in one-fifth of its U.S. restaurants.

McDonald’s says it has been working closely with federal food safety regulators since late last week, when it was alerted to the potential outbreak. The U.S.

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. One person in Colorado died, and 10 people were hospitalized. State and local public health officials are interviewing people about the foods they ate in the week before they got sick.

Of the 18 people interviewed, all reported eating at McDonald’s and 16 people reported eating a beef hamburger. Twelve reported eating a Quarter Pounder. McDonald's says the investigation has been complicated by the scope of the problem.

The company said it serves 1 million Quarter Pounders in the affected area.