US federal health officials have concluded an investigation into an E. coli outbreak linked to McDonald's Quarter Pounders, pinpointing the sliced onions in the burgers as the likely culprit. The Food and Drug Administration (FDA) stated that this outbreak has resulted in 104 infections across 14 states, with 34 people hospitalized and 1 fatality. State and federal health officials announced on Tuesday that E. coli tests on a recalled yellow onion sample and an environmental sample from a grower came back positive, although neither matched the strains found in the reported cases. Citing epidemiological and traceback evidence, the FDA indicated that the recalled yellow onions may be the source of this outbreak. Health officials at the state and local levels interviewed infected individuals, many of whom recalled consuming McDonald's products with sliced onions. On Tuesday, McDonald's stock price rose slightly, while the US large cap market index experienced a slight decline.