McDonald’s is responding to a recent E. coli outbreak.

The fast food chain temporarily removed its Quarter Pounder burger from the menu of restaurants in affected areas—mostly Colorado and Nebraska—on Oct. 22 following a food contamination that has killed one person.

“Serving customers safely in every single restaurant, each and every day, is our top priority and something we’ll never compromise on,” McDonald's North America Chief Supply Chain Officer Cesar Piña shared in an internal message obtained by E! News. “It is why we are taking swift and decisive action following an E. Coli outbreak in certain states.”

While the US Centers for Disease Control and Prevention (CDC) has not confirmed the affected foods, the burger chain believes onions are the ingredient that was contaminated with E. coli, a bacteria found in the intestines of people and animals.

“The initial findings from the investigation indicate that a subset of illnesses may be linked to slivered onions used in the Quarter Pounder and sourced by a single supplier that serves three distribution centers,” Piña continued. “As a result, and in line with our safety protocols, all local restaurants have been instructed to remove this product from their supply and we have paused the distribution of all slivered onions in the impacted area.”