Since November 11, 2003, a major outbreak of Legionnaires' disease had been raging in the Lens region. As of December 31, 2003, there were 52 cases in the region, including 6 deaths. Epidemiological and environmental investigations conducted by government agencies under the authority of the Prefect of Pas-de-Calais had identified and shut down a source of contamination: the cooling towers of the Noroxo company located in the town of Harnes. During this shutdown and despite the cleaning carried out before the company resumed operations, cases continued to appear, and the existence of another source of contamination was then considered. The mission focused on reviewing all investigations conducted by government agencies and analyzing the factors that could be the cause(s) of this outbreak. To this end, the measures taken by the Noroxo company were examined in particular, and models of potential dissemination from this company were developed. Preliminary reports were submitted to the Prefect of Pas-de-Calais, the Director General of Health, and the Director of Pollution and Risk Prevention on January 16, 2004, and March 9, 2004. This document reports on and comments on the methods used for the epidemiological and environmental investigations, the modeling performed to assess the atmospheric dispersion of potentially contaminated aerosols in the Lens region, and the evaluation of Legionella risk management at Noroxo. The results of these investigations, modeling, and assessments are then summarized and discussed to clarify the various factors that may have contributed to the onset and progression of this outbreak. Finally, this report sets forth recommendations regarding various areas of research and study that could help improve understanding of Legionella outbreaks and enhance Legionella risk management in industrial settings.