This U.S. Coast Guard safety alert addresses critical concerns uncovered during an ongoing marine casualty investigation and should be of vital interest to Ship Builders, Classification Societies, Owner / Operators and others involved with vessel operations. A machinery space fire onboard a relatively new vessel was effectively responded to and extinguished by the vessel’s quick response team firefighters using portable extinguishing equipment. However, before it was declared completely extinguished and approximately five hours after the fire started, the master of the vessel made the decision to release CO2 from the vessel’s fixed firefighting system. It failed to operate as designed.