At least 49 people in 10 states have been sickened. One person has died.

Health officials are investigating an E. coli outbreak linked to McDonald’s Quarter Pounders in multiple states, the Centers for Disease Control and Prevention said Tuesday.

It’s not yet known which ingredient in the hamburgers is making people sick. Slivered onions and quarter-pound beef patties, both of which are specifically used for Quarter Pounders, are being looked at.

At least 49 people in 10 states have been sickened. One person, an older adult in Colorado, has died.

Ten people have been hospitalized, including a child who developed a kidney disease called hemolytic uremic syndrome.