This report demonstrates how carbon dioxide (CO2) may be a potent weapon in murder-suicide, where the death scene offers virtually no clues as to the lethal modality and the autopsy findings are nonspecific. Four bodies were discovered in an apartment in midsummer 2012 in Berlin, Germany. The bodies were those of a father (a 69-year-old business consultant), his wife (aged 26-years), and two sons (aged 3 and 6 years, respectively). The police found the wife and two sons lying in their beds and the husband in a supine position on the floor with a plastic bag over his head tied loosely around his neck with a rope. A 500 g single-use CO2 cylinder was standing on the floor. The container was almost empty and according to the label had been sold as a CO2-fertilizer for aquarium plants. Two synthetic inhalation face masks and tubing were also found, which tested positive for the DNA of all four deceased family members. It is hypothesized that the husband placed an inhalation mask over the mouths and noses of his wife and children while they were sleeping. Inhalation of pure CO2 ensured their rapid unconsciousness due to hypercapnia and severe anoxia. The rapid increase in CO2 concentration would render a victim helpless, with no time to wake and defend themselves, or others. The proximate cause of death in all cases was attributed to CO2 intoxication, based on the scene findings, the reconstructed sequence of events, the autopsy, and results of toxicological studies.