McDonald's is investing $100 million to bring customers back to stores after an outbreak of E. coli food poisoning tied to onions on the fast-food giant's Quarter Pounder hamburgers.

The investments include $65 million that will go directly to the hardest-hit franchises, the company said.

The U.S. Centers for Disease Control and Prevention has said that slivered onions on the Quarter Pounders were the likely source of the E. coli. Taylor Farms in California recalled onions potentially linked to the outbreak.

Colorado reported at least 30 cases; Montana reported 19; Nebraska, 13; and New Mexico, 10. The illnesses were reported between Sept. 12 and Oct. 21. At least 104 people got sick and 34 were hospitalized, according to federal health officials. One person died in Colorado and four people developed a potentially life-threatening kidney disease complication.

The Food and Drug Administration has said that “there does not appear to be a continued food safety concern related to this outbreak at McDonald’s restaurants.”

But the outbreak hurt the company's sales.

Quarter Pounders were removed from menus in several states in the early days of the outbreak. McDonald’s identified an alternate supplier for the 900 restaurants that temporarily stopped serving the burgers with onions. Over the past week, McDonald's resumed selling Quarter Pounders with slivered onions nationwide.