McDonald's Quarter Pounder
A Food and Drug Administration spokesperson announced the agency is investigating Taylor Farms as a potential source of the E. coli outbreak linked to hamburgers. Scott Olson/Getty Images

McDonald's, on Thursday, identified Taylor Farms as the supplier of onions used in the company's Quarter Pounder burgers, which have been removed from menus in several states after a deadly E. coli outbreak.

Following McDonald's, other prominent U.S. fast-food chains, including Burger King and Taco Bell, have removed fresh onions from their menu items.

A Food and Drug Administration spokesperson announced Thursday that the agency is investigating Taylor Farms as a potential source of the E. coli outbreak linked to hamburgers, NBC reported.

According to McDonald's spokesperson, the raw onions used in their restaurants were sourced from a single supplier and processed at one facility. The onions are sliced and packaged as raw vegetables in individual bags before being distributed to restaurants.

The FDA and the Centers for Disease Control and Prevention (CDC) had earlier pinpointed slivered onions in the burgers as the likely source of contamination.

In response to the outbreak, Taylor Farms has recalled several yellow onion products due to "potential E. coli contamination," according to a notice from U.S. Foods, which distributes Taylor Farms' products to various restaurants.

Taylor Farms is the exclusive onion supplier for the affected McDonald's locations in Colorado, Kansas, Utah, Wyoming, and portions of Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico, and Oklahoma.

Food distributors, including Sysco Corp, have been notifying their customers, such as restaurants, about the recall. They have advised establishments to immediately stop using and dispose of the affected products.

"Yellow onions were sold to additional food service customers. Customers who received recalled onions have been directly notified of the recall," an FDA spokesperson said.

On Thursday, Burger King's parent company, Restaurant Brands International, and Yum Brands, which owns KFC, Pizza Hut, and Taco Bell, announced the removal of fresh onions from their menu.

Nearly 5% of Burger King outlets that receive supplies from Taylor Farms have halted onion service, but a company spokesperson stated that they have not reported any illnesses or been contacted by health authorities.

Yum Brands indicated that the decision to remove onions is being made "out of an abundance of caution."

Meanwhile, the U.S. health regulator announced that it is collaborating with federal and state partners, as well as the companies involved, to investigate whether onions are the source of the outbreak, and if McDonald's beef patties may be affected. However, E. coli is eliminated in properly cooked beef, unlike in McDonald's Quarter Pounder, which is served with raw, slivered onions.

The CDC said, as of Wednesday, at least 49 people had been sickened with E. coli infections linked to the outbreak. One older adult has died, and 10 other people, including a child suffering from hemolytic uremic syndrome, have been hospitalized. The E.coli outbreak was first reported in late September.