Ten people have been hospitalized and an elderly person in Colorado has died after E. coli infections linked to McDonald’s Quarter Pounder hamburgers in 10 states, the U.S. Centers for Disease Control said Tuesday.

The company’s shares fell about 9% in after-hours trading.

All respondents reported eating at McDonald’s before the onset of their illness, and specifically mentioned eating a Quarter Pounder hamburger, according to the CDC.

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

McDonald’s has proactively removed the onion slices and beef patties used in its quarter-pound hamburgers from stores in affected states while the investigation continues, the company told the CDC.

McDonald’s did not immediately respond to a Reuters request for comment.

According to the CDC, 49 people from 10 states have become ill with the same strain of E. coli. Most of the sick come from Colorado or Nebraska.

Symptoms for E. coli include severe stomach cramps, diarrhea, and vomiting.

According to the CDC, Quarter Pounders may be temporarily unavailable in some states.

In 2015, burrito chain Chipotle, which opened a new location, experienced lost sales and reputational damage due to E. coli outbreaks in several states.