A deadly E. coli outbreak linked to McDonald‘s Quarter Pounders has led to 75 cases in 13 states, the Centers for Disease Control and Prevention said on Friday, as it investigates the source of the spread. 

The outbreak has led to 22 hospitalizations and one death.

The CDC gave the new tally of people affected three days after the agency announced the outbreak on Tuesday, citing 49 cases and one death across 10 states. Shares of the restaurant chain are down 5% since Tuesday.

Quarter Pounder hamburgers are a core menu item for McDonald’s, raking in billions of dollars annually.

Health officials are closely examining the slivered onions used in the Quarter Pounder as a likely contaminant. McDonald’s has instructed restaurants in the affected area to remove slivered onions from their supply, and has paused the distribution of that ingredient in the region.

McDonald’s identified California-based produce giant Taylor Farms as the supplier for the sliced onions the company removed from its supply chain. Taylor Farms has issued a recall on four raw onion products due to potential E. coli contamination.

But federal agencies are also investigating the Quarter Pounder’s beef patty as a potential culprit.

As the CDC and other federal agencies trace cases and work to contain the outbreak, McDonald’s has pulled Quarter Pounders from restaurants in the affected areas. Around a fifth of McDonald’s U.S. restaurants are not selling Quarter Pounder burgers.