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

Shares of the company were down about 9% after the bell.

Everyone interviewed has reported eating at McDonald’s before their illness started, and most specifically mentioned eating a Quarter Pounder hamburger, according to the CDC.

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

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.

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

Forty nine people from 10 states have fallen sick from the same strain of E. coli, the CDC said. Most sick people are from Colorado or Nebraska.

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

Quarter Pounders in some states may be temporarily unavailable, according to the CDC’s statement.

In 2015, burrito chain Chipotle, opens new tab saw its sales battered and reputation hit due to E.coli outbreaks in several states.