Firefighters were called to a house next door to where a couple were later found unconscious and not breathing but left the scene after finding no sign of a blaze.
Joe Morgan and his wife, Keli, were rushed to hospital from their home in Maghull, Merseyside, on Saturday evening but later died.
An interim incident report released by Merseyside Fire and Rescue Service revealed crews were called to the adjoining property in Bridge Farm Drive earlier on the same evening after reports of smoke coming from a light fitting in the living room.
Firefighters were said to have performed a "thorough investigation" for 25 minutes but found no sign of smoke or any indication of a fire and left the scene.
Shortly afterwards a second call was received from the same property with reports that the bathroom was filling with smoke.
Firefighters returned to the address and quickly identified the source of the smoke as the home next door where the Morgans lived.
The interim report indicated that a chip pan left unattended on a gas ring in the kitchen was the cause of the fire. A smoke alarm was fitted at the property but was not functioning.
It added that it is believed a small amount of smoke had travelled through the ceiling void into the house next door and out through the light fitting in their living room.
The smoke was visible for a short period before clearing before the arrival of firefighters, the report said.
Group manager Guy Keen said: "Our thoughts go out to the friends and relatives of those who died in this tragic incident.
"We would urge all members of the public to ensure working smoke alarms are fitted in their home and that they are checked regularly to ensure they work."