One person died and dozens fell ill from E. coli infections linked to McDonald’s Quarter Pounder hamburgers in 10 states, led by Colorado, where 26 people were sickened, the U.S. Centers for Disease Control said on Tuesday.

The E. coli outbreak, linked to one of McDonald’s most popular menu items, has sickened 49 people and sent 10 to the hospital, officials say.

The strain involved, E. coli O157:H7, can cause serious illness and was the source of a 1993 outbreak that killed four children who ate undercooked hamburgers at Jack in the Box restaurants.

Shares of the world’s largest fast-food chain were down about 6% in extended trading. A livestock trader said the outbreak also could pressure U.S. cattle futures on Wednesday by threatening demand for beef.

Everyone interviewed as part of an investigation into the outbreak has reported eating at McDonald’s before their illness started, and most mentioned eating a Quarter Pounder hamburger, according to the CDC.

The specific ingredient linked to the illness has not been identified but investigators are focused on fresh, slivered onions and fresh beef patties, the CDC said.

Most of the illnesses were reported in Colorado and Nebraska.

“The initial findings from the investigation indicate that a subset of illnesses may be linked to slivered onions used in the Quarter Pounder and sourced by a single supplier that serves three distribution centers,” McDonald’s North America Chief Supply Chain Officer Cesar Piña said in a statement.

McDonald’s has proactively removed the slivered onions and beef patties used for the Quarter Pounder hamburgers from stores in the affected states while the investigation continues, the company informed the CDC.

U.S. food safety attorney Bill Marler, who represented a victim in the Jack in the Box outbreak, said more cases of illness could surface. Onions have been linked to prior E. coli O157:H7 outbreaks, he said.

According to Marler, a founder of Marler Clark in Seattle, beef contamination is less common due to food safety measures. “You’d have to have multiple restaurants under-cooking the meat,” he said.

McDonald’s is temporarily removing the Quarter Pounder from restaurants in the impacted areas, including Colorado, Kansas, Utah and Wyoming, it said in a statement, adding it was working with suppliers to replenish supply in the coming week.

Symptoms for E. coli include severe stomach cramps, diarrhea and vomiting. Most people who suffer an infection will start feeling sick three to four days after eating or drinking something that contains the bacteria, Colorado’s public health department said. However, illnesses can start anywhere from one to 10 days after exposure, the department added.

In 2015, burrito chain Chipotle saw its sales battered and reputation hit due to E.coli outbreaks in several states. That outbreak was linked to a different strain of E. coli that typically causes less severe illness than E. coli O157:H7.

In addition to Colorado, the CDC said small clusters of a few people fell ill after eating a Quarter Pounder in Nebraska, Utah and Wyoming. Kansas, Missouri, Oregon, Iowa, Wisconsin and Montana had one illness apiece.