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Infections were reported between Sept. 27 and Oct. 11, in Colorado, Iowa, Kansas, Missouri, Montana, Nebraska, Oregon, Utah, Wisconsin and Wyoming. Colorado has the most cases, 26, followed by Nebraska with nine.

Everyone interviewed in connection with the outbreak had reported eating at McDonald's before falling ill and most mentioned eating Quarter Pounder hamburgers, the CDC said. The U.S. Agriculture Department, the Food and Drug Administration and state health officials are also investigating.

A specific ingredient has not been identified as the cause, but investigators are focused on onions and beef. A preliminary FDA investigation suggests that slivered onions served on the burgers are a likely source of contamination. The USDA is investigating the hamburger patties.

In a statement, McDonald's officials said that initial findings suggest that some illnesses are linked to onions sourced from a single supplier. The company has halted distribution of the slivered onions and temporarily removed the Quarter Pounder from menus in the affected states, and also in portions of Idaho, Nevada, New Mexico and Oklahoma.

“We take food safety extremely seriously and it's the right thing to do,” the statement said.

McDonald's Canada said in a statement that none of its restaurants or menu items were affected.

are harbored in the guts of animals and found in the environment. Infections can cause severe illness, including fever, stomach cramps and bloody diarrhea. People who develop symptoms of E. coli poisoning should seek health care immediately and tell the provider what they ate.

The news comes in an already tough year for the Chicago-based McDonald's chain. Its global same-store sales in nearly four years in the second quarter as inflation-weary customers skipped eating out or chose cheaper options. The company responded with a $5 meal deal, which was introduced at U.S. restaurants in late June and was recently . The deal doesn’t include the Quarter Pounder.

McDonald’s shares dropped 9% in after-hours trading Tuesday after the CDC’s announcement.

The type of bacteria implicated in this outbreak, E. coli O157:H7, causes about 74,000 infections in the U.S. each year, leading to more than 2,000 hospitalizations and 61 deaths. Infections are especially dangerous for children younger than 5 and can cause acute kidney failure.

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Associated Press writer Dee-Ann Durbin in Detroit contributed to this story.

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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Science and Educational Media Group. The AP is solely responsible for all content.