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The Minnesota Department of Health identified an outbreak of Legionnaires' disease in a city in northern Minnesota, USA, in April 2023 that continued until chloramine disinfection of the community water system was implemented. Before chloramine disinfection was implemented, Legionella pneumophila was detected in 1 of 16 samples from the drinking water distribution system and in 5 of 10 premise plumbing samples using both cultivation-dependent (Legiolert) and cultivation-independent (digital PCR) assays in this independent investigation. Approximately 11 weeks after disinfection was implemented, all distribution system samples tested negative; however, 1 of 6 Legiolert-tested and 3 of 6 digital PCR-tested premise plumbing samples were positive. After 24 weeks of disinfection, all samples collected from the distribution system and premise plumbing tested negative. Our results show that a community water system supplied by groundwater supported substantial growth of L. pneumophila in premise plumbing and that chloramine disinfection halted the outbreak.