On Wednesday, McDonald’s made efforts to reassure its customers about the safety of its U.S. locations as federal authorities investigated the source of a lethal E. coli outbreak associated with the fast-food chain’s Quarter Pounder hamburgers.

In response to the outbreak, which the U.S. Centers for Disease Control and Prevention (CDC) reported had affected at least 49 individuals across 10 states—resulting in one fatality and 10 hospitalizations—McDonald’s temporarily removed Quarter Pounders from a fifth of its U.S. restaurants on Tuesday.

A preliminary assessment by the U.S. Food and Drug Administration indicated that fresh slivered onions, typically served raw on Quarter Pounder hamburgers, were a probable cause of the contamination. While McDonald’s also includes raw slivered onions on a breakfast sandwich, that particular item is not sold at the affected locations. Other burgers, such as the Big Mac, feature diced, cooked onions instead.

To address the situation, McDonald’s is actively seeking a new regional supplier for fresh onions. In the interim, Quarter Pounders have been taken off the menus in Colorado, Kansas, Utah, Wyoming, and parts of Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico, and Oklahoma.