During a flushing operation on a settling tank of the sewer system, 3 sewermen died, ranging from 22 to 44 years of age, and another was severely injured, following a release of hydrogen sulphide (H2S). Two times per year, 4 employees of a drainage and road construction company clean the settling tank of the located in a district. The operation consists in sludge and other waste from the 5-meter deep, 30 m³ settling tank into trucks. The company has worked under contract for the city more than twenty years. According to the company, this preventive maintenance operation, which had begun at 9.30 am, was designed to ensure the proper flow of wastewater in the sewer. Around 10 am, 3 of the workers were immediately intoxicated (according to a member of the rescue team) as the probably reached a pocket of H2S, a highly toxic gas formed from the decomposition of organic materials. The fourth worker, the 48-year old father of one of the dead employees, was located a slight distance from the others and was seriously injured and transported to the hospital. Alerted by a passer-by, roughly fifty firemen and twenty vehicles came to the scene, and were joined by 4 paramedic teams (SAMU). A judicial inquiry and an inquiry by the labour inspectorate were conducted to determine if all the protocols for this type of intervention had been respected. The company’s management indicated that wastewater systems operators receive training regarding interventions in confined atmospheres, that they have atmosphere testers and gas masks. An autopsy was ordered by the court to know the exact cause of death of the three workers.