Due to the E. coli outbreak, McDonald's is removing Quarter Pounder from the menu at one in five restaurants

McDonald's has stopped selling Quarter Pounder hamburgers in about a fifth of its U.S. restaurants as federal health officials conduct an investigation E. coli outbreak That sickened nearly 50 people in 10 states and killed one.

The fast-food chain sold about a million Quarter Pounders during the period the illnesses occurred, a McDonald's spokesman said Wednesday. The company believed that more people would have become ill if the contamination came from the patties themselves, adding that the onions used in the burgers may be the cause.

The burgers sold by McDonald's are cooked at 175 degrees, more than the 160 degrees needed to kill the E. coli bacteria, the spokesman said.

However, the chopped onions used as a topping on Quarter Pounder burgers sold in the affected region were uncooked and came from a single supplier. “If this is the source, it would be the first time that onions have been a carrier of this strain of E. coli,” the spokesman said.

The U.S. Centers for Disease Control and Prevention on Tuesday linked McDonald's Quarter Pounder hamburgers to an outbreak of E. coli that has sickened dozens of people, mostly in Colorado and Nebraska.