Restaurant chains in Colorado, including Illegal Pete's and Taco Bell, also removed onions from their menus following the recall. A spokesperson for Yum! Brands, the parent company of Taco Bell, said Pizza Hut and KFC have also removed fresh onions from their menus at select restaurants. There is no evidence of E. coli illnesses associated with these restaurants.

Until now, it wasn't clear where the McDonald's onions came from – neither the restaurant chain nor health authorities had publicly said where the onions were grown or whether they were supplied to other restaurants.

A McDonald's spokesman said Wednesday that the raw onions are sourced from a single supplier and processed at a single facility. They are sliced ​​at the facility and packed into individual bags as raw vegetables and then distributed to restaurants.

A spokesman 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 E. coli strain found 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 rise.

Meanwhile, McDonald's is already facing lawsuits.

On Thursday, Ron Simon & Associates, a food safety law firm, announced that it had filed a second lawsuit, this time on behalf of Clarissa DeBock, a Nebraska resident who said she had a Quarter Pounder on Sept. 18 Bought at local McDonald's. Days later, according to the lawsuit, DeBock became ill and had to see a doctor, where she tested positive for O157:H7.

The firm's first lawsuit was filed on behalf of Colorado resident Eric Stelly. Stelly said he suffered from gastrointestinal distress after eating a burger at McDonald's.

Ron Simon, the firm's managing partner, told NBC News that he is currently representing 15 families claiming to be affected, with patients ranging in age from 20 to 60.

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