California. A deadly E. coli outbreak linked to McDonald's Quarter Pounders has affected at least 75 people across 13 states, federal health officials reported on Friday.

The Centers for Disease Control and Prevention stated that 22 individuals have been hospitalized, with two developing severe kidney complications and one confirmed death.

While the source of the outbreak remains unidentified, the US Food and Drug Administration is investigating slivered onions and beef patties used in the burgers.

McDonald's has indicated that Taylor Farms, a California-based produce grower, supplied the recalled yellow onions potentially linked to the contamination. In response to the outbreak, McDonald's removed the affected burger from menus in several states on Tuesday.

McDonald’s worked Wednesday to reassure customers that its US restaurants are safe as federal investigators tried to pinpoint the cause of a deadly E. coli outbreak linked to the fast-food giant’s Quarter Pounder hamburgers.

McDonald’s pulled Quarter Pounders from one-fifth of its U.S. stores Tuesday as a result of the outbreak, which the U.S. Centers for Disease Control and Prevention said had sickened at least 49 people in 10 states. One person died and 10 were hospitalized, according to the CDC.

A preliminary investigation by the U.S. Food and Drug Administration suggested fresh slivered onions that are served raw on Quarter Pounder hamburgers were a likely source of the contamination. McDonald’s also serves raw, slivered onions on one of its breakfast sandwiches, but that sandwich isn’t available at the impacted stores. Other burgers, like the Big Mac, use diced, cooked onions.

McDonald’s said it was searching for a new regional supplier for fresh onions. In the meantime, Quarter Pounders were removed from menus in Colorado, Kansas, Utah, Wyoming, and portions of Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico, and Oklahoma.