Their initial investigation noted 49 cases of E. coli throughout 10 U.S. states (Colorado, Kansas, Utah, Wyoming, and portions of Idaho, Iowa, Missouri, Montana, Nebraska, New Mexico, and Oklahoma).

The Centers for Disease Control and Prevention (CDC) at the time did not confirm the origin, but the government agency suspected it was either slivered onions or beef patties.

[5] On October 22, McDonald's stores in the affected states temporarily removed the Quarter Pounder from the menu or stopped serving it with the specific slivered onions and beef patties.

A second update on the investigation was published on October 30, stating that epidemiologic and traceback information shows that fresh, slivered onions were the likely source of illness in this outbreak.

[10][11] On October 23, a man who fell ill after eating a sandwich at the chain's Greeley, Colorado, location filed a lawsuit, one of the first in the case.