Shares of McDonald’s (MCD) fell as much as 10% in after-hours trading on Tuesday after the Centers for Disease Control and Prevention said the company’s quarter pounder burgers had been linked to an E. coli outbreak in some states, with the most diseases in Colorado. and Nebraska.

“This is a rapidly evolving outbreak investigation,” the CDC wrote on its website. “Most sick people report eating McDonald’s Quarter Pounder burgers and investigators are working quickly to confirm which food ingredient is contaminated.”

The CDC said McDonald’s has stopped using fresh-cut onions and quarter-pound beef patties in certain states while a source of the disease has been confirmed.

One person has died from the outbreak, the agency said, and 10 hospitalizations have been reported in 10 states.

In an internal memo that McDonald’s shared on its website Tuesday evening, Cesar Piña, McDonald’s Chief Supply Chain Officer for North America, said the company is taking “swift and decisive action” and noted that the investigation’s initial findings “indicate that a subset of diseases may be linked to chopped onions used in the Quarter Pounder that come from a single supplier serving three distribution centers.”

“As a result, and in line with our safety protocols, all local restaurants have been instructed to remove this product from their offerings and we have halted the distribution of all chopped onions in the affected area,” the company said.

The menu item is being temporarily removed from restaurants in the affected areas, including Colorado, Kansas, Utah and Wyoming, as well as parts of Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico and Oklahoma. All other menu items are available.

Fellow fast food giant Chipotle (CMG) experienced its own E. coli outbreak in 2015, along with a norovirus outbreak. As a result, the company was forced to temporarily close 43 locations in Washington and Oregon.

The CDC declared the outbreak over in February 2016, with the company aggressively revamping its food preparation practices.

But it took years for the Mexican fast food giant to rebuild brand trust and recover its share price as a result of the crisis. Shares are up more than 500% since 2016.

Alexandra Canal is a senior reporter at Yahoo Finance. Follow her on X @allie_canal, LinkedIn, and email her at [email protected].

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