Authorities said the former tenants, a woman and her adult son, had previously blocked up a ventilation outlet, possibly causing the deadly gas to flow into the Köpenick apartment when the heater was used.
They told authorities that they stuffed the exhaust pipe leading from a gas boiler several years ago to prevent drafts and cold.
Carbon monoxide is a colourless, tasteless and odourless gas that is highly toxic to humans in significant quantities. It can be produced by old-fashioned heating systems and prove deadly when it leaks into confined spaces.
Firemen discovered the bodies of a 27-year-old mother and her four young children, as well as the woman's 40-year-old partner, after breaking down the front door Monday. They were called by an acquaintance of the family.
Police and prosecutors have said more investigation is needed. They are further examining the apartment's gas boiler.