Plug the exhaust vent and the exhaust gas can be forced out of the furnace into the furnace room instead. Sometimes the boiler has what looks like a damper in the stove pipe .... and exhaust  gas could be forced out there if the outside opening were blocked .... or even partially blocked. The furnace room can then begin to fill with toxic gas. How does it fill up the rest of the house? If you turn on a bathroom fan any where in the house, turn on the exhaust fan over a cook stove or even run the clothes dryer .... air is being pushed out of the house ... when that happens a vacuum is created and make up air is drawn into the house. If no source of make up air is provided for .... it can draw from the furnace room .... because the furnace generally has a path to outside air. Over time this could slowly fill the house with exhaust gas. One report said the occupants may have had experienced headaches for some time before event.