The majority of the McDonald’s E. coli cases in Colorado originated from Mesa County, officials said Wednesday.

As of Tuesday, 26 cases of Shiga toxin-producing E. coli (STEC) have occurred in Colorado. Eighteen of those cases have come from Mesa County, with one person dying from the infection, according to a Wednesday news release from Mesa County Public Health (MCPH) — in coordination with the Colorado Department of Public Health and Environment (CDPHE) and the Centers for Disease Control and Prevention (CDC).

No details about the victim were released. The Denver Gazette reached out to the Mesa County Coroner, but had not heard back as of press time.

All cases in Colorado have been within nine counties: Arapahoe, Chaffee, El Paso, Gunnison, Larimer, Mesa, Routt, Teller and Weld.

But a spokesperson with CDPHE told The Denver Gazette that it’s expected that all McDonald’s locations in Colorado received the infected ingredients.

The outbreak, involving 49 documented cases nationwide, was reportedly scattered over 10 states, according to the CDC. Early information from the U.S. Food and Drug Administration tied the problem to the “fresh-slivered onions” on the McDonald’s Quarter Pounder hamburger. The specific onions are not used on other menu items.

In response, McDonald’s has pulled the hamburger and its associated ingredients off the menu in Colorado, Kansas, Utah and Wyoming, as well as parts of Idaho, Iowa, Missouri, Montana, Nebraska, Nevada and Oklahoma, according to McDonald’s North America Chief Supply Chain Officer Cesar Piña.

The onions trace back to three distribution centers, all serviced by the same supplier.

The United States Department of Agriculture (USDA) is also looking into the beef patties on the Quarter Pounder hamburgers.

MCPH officials said they were working to monitor the situation, with epidemiologists conducting interviews with residents who tested positive. Mesa County’s Environmental Health Team has also conducted investigations at specific restaurants and is working with restaurant management to ensure safety protocols are in place, according to the news release.

“We have local experts on hand with an efficient system in place to handle these situations. This helps with the speed of investigations to help prevent additional people from becoming sick,” Erin Minnerath, deputy director at Mesa County Public Health, said in the release. “The system remains in place for residents in Mesa County to have food that’s safe to eat.”

State and local public health officials across the 10 affected states have been interviewing people about the foods they ate in the week before they got sick. All 18 people interviewed reported eating at McDonald’s and 16 people reported eating a beef hamburger. Twelve reported eating a Quarter Pounder.

McDonald’s noted that the investigation is difficult because it serves one million Quarter Pounders in the affected areas every two weeks. McDonald’s also said this strain of E. coli isn’t usually found in onions.

The company said Wednesday it is searching for a new regional supplier for fresh onions, and that Quarter Pounders have been removed from menus in one-fifth of its U.S. restaurants.

McDonald’s corporate media relations, when asked about the Mesa County cases, replied to The Denver Gazette Wednesday: “McDonalds maintains strict food safety standards and protocols and take our approach to food safety management extremely seriously.” No specific questions about Mesa County, or what corporate is doing to help affected franchise owners, was included.

The spokesperson directed the inquiry to the company’s food safety website.

“I think this is going to be a big outbreak and the numbers are going to grow,” Jory Lange, a national food poisoning lawyer, told The Denver Gazette. “It’s already larger than you typically see in an E. coli outbreak.”

Lange noted that there’s typically around 10-to-20 cases around this period of an outbreak, with potential delays in reporting to the CDC due to the delay in symptoms and the time of testing. There are already 49, with one person developing hemolytic uremic syndrome (HUS) — a complication of E. coli that blocks the kidney, potentially leading to failure.

The death in Mesa County in connection to the infection is rare, but CDPHE said that the man was in his 80s and had other medical conditions.

The amount of states may also grow from 10 as reports come in and people develop symptoms, Lange said.

The incubation period for E. coli is only a couple of days, and the first reports were on Sept. 27. Illness would quickly affect those that have contacted it, Donald Schaffner, a food safety expert at Rutgers University, said.

“If you ate these burgers in September and now it’s the middle of October and you didn’t get sick, you’re probably OK,” he said.

Regarding potential lawsuits, Lange said there is still a waiting period as investigators find out where the cause lies.

McDonald’s is ultimately liable because they sold the product, but if the cause is the onions, liability will trace back further, according to Lange.

“If the onions are part of this, McDonald’s will be liable and as far back as those onions can be traced, as close to the farm as they can get, everyone in that chain of supply will be liable,” he said.

The Associated Press contributed to this report.