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In 2021, the U.S. Food and Drug Administration (FDA), the Centers for Disease Control and Prevention (CDC), and state and local health and regulatory partners investigated an outbreak of Salmonella enterica serovar Oranienburg infections linked to bulb onions from Mexico, resulting in 1040 illnesses and 260 hospitalizations across 39 states, the District of Columbia, and Puerto Rico. The Kansas Department of Agriculture recovered the outbreak strain of Salmonella Oranienburg from a sample of condiment collected from an ill person's home. The condiment was made with cilantro, lime, and onions, but, at the time of collection, there were no onions remaining in it. FDA conducted traceback investigations for white, yellow, and red bulb onions, cilantro, limes, tomatoes, and jalapeño peppers. Growers in the state of Chihuahua, Mexico, were identified as supplying the implicated onions that could account for exposure to onions for all illnesses included in the traceback investigation, but investigators could not determine a single source or route of contamination. FDA collected product and environmental samples across the domestic supply chain but did not recover the outbreak strain of Salmonella. Binational collaboration and information sharing supported Mexican authorities in collecting environmental samples from two packing plants and onion, water, and environmental samples from 15 farms and firms in Chihuahua, Mexico identified through FDA's traceback investigation, but did not recover the outbreak strain.. Distributors of the implicated onions issued voluntary recalls of red, yellow, and white whole, fresh onions imported from the state of Chihuahua, Mexico. This outbreak showcased how investigators overcame significant traceback and epidemiologic challenges, the need for strengthening the ongoing collaboration between U.S. and Mexican authorities and highlighted the need for identifying practices across the supply chain that can help improve the safety of onions. • An outbreak of Salmonella Oranienburg infections was linked to onions from Mexico. • U.S. and Mexico regulatory partners collaborated on the outbreak investigation. • Traceback and epidemiologic evidence helped identify the source of the onions. • The investigation included traceback, epidemiology, and produce safety challenges. [ABSTRACT FROM AUTHOR]