McDonald’s said Thursday that Taylor Farms is the supplier of the onions served on Quarter Pounders, which were pulled from the restaurant giant’s menus in multiple states following a deadly E. coli outbreak.

The Food and Drug Administration and the Centers for Disease Control and Prevention have homed in on slivered onions served on the hamburgers as the likely source of the contamination.

In a notice to customers Wednesday, distributor U.S. Foods said Taylor Farms announced a recall of four raw onion products out of an abundance of caution because of “potential E. coli contamination.” Customers such as restaurants were urged to stop using and destroy the affected products as soon as possible.

Colorado restaurant chains, including Illegal Pete’s and Taco Bell, also removed onions from their menu following the recall. A spokesperson for Yum! Brands, the parent company that owns Taco Bell, said Pizza Hut and KFC at selected restaurants have also removed fresh onions from their menus.There are no signs of E. coli illnesses linked to those restaurants.

An FDA spokesperson said Thursday the agency was probing Taylor Farms as the possible source of the E. coli outbreak linked to hamburgers, adding, “We’re looking at all possible sources.”

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.

Until now, it wasn’t clear where the McDonald’s onions were sourced from — neither the restaurant chain nor public health officials had said publicly where the onions were grown or whether they were sent to other restaurants.A McDonald’s spokesperson said Wednesday that the raw onions were sourced from a single supplier and processed at a single facility. They aresliced and packaged at the facility as raw vegetables in individual bags and then distributed to restaurants.

A spokesperson for Taylor Farms did not immediately respond to a request for comment. According to the company’s website, Taylor Farms is a California-based producer of fresh-cut fruits and vegetables.

The strain of E. coli in the outbreak, called O157:H7, produces a powerful toxin that can damage the lining of the small intestine.

Health officials said Wednesday that they expect the number of cases to grow.

In the meantime, McDonald’s is already facing lawsuits.

On Thursday, Ron Simon & Associates, a food safety law firm, said it filed a second lawsuit, this time on behalf of Clarissa DeBock, a Nebraska resident who said she purchased a Quarter Pounder from a local McDonald’s on Sept. 18. Days later, DeBock became ill and was forced to seek medical attention where she tested positive for O157:H7, according to the lawsuit.

The first lawsuit from the firm was on behalf of Colorado resident Eric Stelly. Stelly said he suffered from gastrointestinal symptoms after having eaten a burger at McDonald’s.

Ron Simon, the managing partner of the firm, told NBC News that he is now representing 15 families who claim to be affected, with patients ages 20 to 60.

“They’re in different states and different communities with different outcomes,” he said.