(Reuters) – Ten people have been hospitalized and one elderly 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.

The company’s shares fell about 9% after the bell.

All interviewees reported eating at McDonald’s before their illness began, most specifically eating a Quarter Pounder hamburger, according to the CDC.

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

McDonald’s has proactively removed the sliced ​​onions and beef patties used in its quarter pounder burgers from stores in affected states as the investigation continues, the company told the CDC.

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

Forty-nine people from ten states have been sickened by the same strain of E. coli, the CDC said. Most of the sick people come from Colorado or Nebraska.

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

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

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

(Reporting by Sneha SK; Editing by Sriraj Kalluvila)