Onions recalled last month, linked to a major outbreak of E. coli from McDonald’s Quarter Pounder burgers, have been classified as in the highest risk category by the U.S. Food and Drug Administration (FDA).

Taylor Farms on Oct. 22 issued a recall for thousands of cases of shaved yellow onions to McDonald’s restaurants and other foodservice customers in multiple states due to possible E. coli contamination. In the first few days that followed, the burgers were removed from menus in several states, with the reported illnesses occurring between September 12 and October 21.

The FDA on Thursday issued a Class I risk rating for many of the affected onions, including those distributed to food service facilities in Colorado, Kansas, Missouri, Nebraska, New Mexico and Utah.

A Class I risk rating means that there is a “reasonable likelihood that use of or exposure to an infringing product will cause serious adverse health effects or death.” There are three classifications in total.

Newsweek contacted Taylor Farms for comment outside normal business hours.

In the onion recall notice, the distributor urged customers to “stop using the affected product as soon as possible” and “destroy” it.

As of Nov. 13, the number of people infected with the bacteria linked to chopped onions on Quarter Pounder burgers had risen to 104 in 14 states, according to the U.S. Centers for Disease Control and Prevention (CDC).

Of these, 34 people were confirmed to have been hospitalized, one person in Colorado had died and four others had developed potentially life-threatening kidney complications.

The state with the highest number of reported E. coli infections is Colorado, with at least 30 cases, followed by 19 in Montana, 13 in Nebraska, 10 in New Mexico, eight in Utah and Missouri, six in Wyoming, three in Kansas, two in Michigan and one case each in Iowa, North Carolina, Oregon, Washington and Wisconsin.

“The actual number of sick people in this outbreak is likely much higher than the reported number, and the outbreak may not be limited to states with known illnesses. This is because many people recover without medical care and are not tested for E. coli,” the CDC said in a statement Wednesday. “Additionally, recent illnesses may not yet be reported because it typically takes three to four weeks to determine whether a sick person is part of an outbreak.”

Tom Skinner, a public affairs official at the CDC, previously shared this Newsweek that “interviews conducted by public health officials with those who were sick with E. coli and interviews with others who did not become ill led officials to suspect McDonald’s quarter pounders as the source.”

FDA testing of recalled onions detected an E. coli strain with a harmful toxin in one onion sample, but officials noted it did not match the strain responsible for the reported illnesses. Further sample results are still pending.

The onions used in these McDonald’s locations have now passed their expiration date and are no longer in circulation. “At this time, there does not appear to be any ongoing food safety concerns associated with this outbreak at McDonald’s restaurants,” the FDA said.

The strain of bacteria involved in this outbreak is responsible for about 74,000 infections, more than 2,000 hospitalizations and 61 deaths in the United States each year, according to CDC data.

E. coli is a type of bacteria that normally lives in the intestines of people and animals, but certain strains can cause serious foodborne illness. While most E. coli strains are harmless, the type involved in this outbreak – known as Shiga toxin-producing E. coli – can cause serious infections and potentially life-threatening complications.

Symptoms typically appear within one to two days after consuming contaminated food and include fever, vomiting, diarrhea or bloody diarrhea, and signs of dehydration, such as low urine output, increased thirst, and dizziness. The infection can lead to serious kidney damage, especially in children under 5 years of age, for whom E. coli poisoning requires emergency medical attention.