Over a 14-day period, four patients developed sepsis within hours of undergoing endoscopic procedures at a gastroenterology outpatient clinic in Geneva (Switzerland), triggering an urgent epidemiological investigation upon notification to authorities. The case clustering raised concerns about a iatrogenic source, prompting a coordinated public health response. This report outlines the investigation led by the Geneva Health Authorities in collaboration with the medical team, emergency care providers, infection prevention specialists, and the bacteriology laboratory at Geneva University Hospitals. A multidisciplinary team, including an epidemiologist, a pharmacist and an infection prevention specialist, conducted an on-site investigation the day after the outbreak was identified, highlighting a probable extrinsic contamination of propofol by Escherichia coli.