Fresh, slivered onions served on Quarter Pounders and other menu items from McDonald's are the likely source of an E. coli outbreak in the United States, said the U.S. Centers for Disease Control and Prevention (CDC) on Wednesday.

A total of 90 cases of people sickened by E. coli had been reported across 13 U.S. states as of Wednesday, including 15 new cases, according to the latest CDC data. Twenty-seven of the people were hospitalized and one has died.

The U.S. Food and Drug Administration (FDA) and McDonald's have confirmed that Taylor Farms was the supplier for the affected locations, and it has since recalled several batches of yellow onions produced in a Colorado facility.

The FDA on Wednesday said it had initiated inspections at a Taylor Farms processing center in Colorado, a state where 29 people have fallen ill due to the outbreak. An onion grower in Washington state is also being investigated, the FDA added.

The CDC said more illnesses have been reported, but the cases originated from before McDonald's and Taylor Farms took action to remove onions from food service locations.

Due to the product actions taken by both companies, the CDC said it believes the risk to the public is very low.

The FDA noted that symptoms of E. coli begin anywhere from a few days after consuming contaminated food to up to nine days later.

As of Wednesday, the outbreak had affected Colorado, Kansas, Utah, Wyoming, Iowa, Missouri, Montana, Nebraska, New Mexico, Oregon, Wisconsin, Washington and Michigan.

McDonald's said that it would resume selling Quarter Pounders burgers this week after it temporarily took the item off the menu in a fifth of its 14,000 U.S. restaurants that were impacted.

On Tuesday, executives of McDonald's brushed off any potential sales hit from the outbreak, with CEO Chris Kempczinski apologizing to customers and adding that he was "confident in the safety of eating at McDonald's."

E. coli are bacteria found in many places, including in the environment, foods, water, and the intestines of people and animals.

Most E. coli are harmless and are part of a healthy intestinal tract. But some E. coli can make people sick with diarrhea, urinary tract infections, pneumonia, sepsis and other illnesses, according to the CDC.

(With input from Xinhua and Reuters)