The investigation into the E. coli outbreak in McDonald's Quarter Pounders that has killed at least one person and sickened nearly 50 others increasingly points to the slivered onions served on the hamburgers.

McDonald’s worked Wednesday to reassure customers that its U.S. restaurants are safe as federal investigators tried to pinpoint the cause of a deadly E. coli outbreak linked to the fast food giant’s Quarter Pounder hamburgers.

An E. coli outbreak linked to McDonald’s Quarter Pounders has led to at least 49 illnesses across 10 states, including one death. Here’s what we know. Quarter Pounders made people sick The US Centers for Disease Control and Prevention issued a food safety alert Tuesday warning that dozens of people reported eating the Quarter Pounder sandwich at McDonald’s before becoming sick.