One person has died and 49 people have become ill from an E. coli outbreak linked to McDonald’s Quarter Pounder hamburgers, the Centers for Disease Control and Prevention said Tuesday.

McDonald’s has now stopped selling Quarter Pounders from locations in several states, mostly in the Mountain West, the CDC said, and has also discontinued the use of slivered onions in all sandwiches in those states as health investigators try to determine which ingredient may be contaminated with the deadly bacteria.

The public health agency called the situation “a fast-moving outbreak investigation” in its news release.

The same strain of bacteria has sickened dozens of people in 10 states, although the CDC said most people were from Colorado and Nebraska. One Colorado resident has died. Ten people were hospitalized, including a child who the health agency said has a complicating illness.

All of those interviewed said they had eaten at McDonald’s recently, and most said they had consumed Quarter Pounders.

— The New York Times