A nine-year-old girl found her mother and her partner dead in separate rooms of their Birmingham home, police said.
The girl dialled 999 after failing to wake Suzanne Van Hagen, 34, and John Worton, 37, in the house in Miranda Close, Frankley, at about 10:00 GMT.
Police said there were no obvious signs of injury to the couple and they were treating the deaths as "unexplained".
They said they were examining if they were poisoned by carbon monoxide.
Police said the girl, who had thought they were asleep, was now being cared for by her grandparents and specially-trained officers.
Ambulance chiefs said paramedics were called following reports two people had been found unconscious in the house. However, they said both had died at the scene.
Officers said Mr Worton was found dead in the lounge of the property, while Ms Van Hagen was discovered near a mattress in a bedroom.
It is understood the girl was taken to hospital for precautionary checks.
Det Ch Insp Wayne Jones said officers were examining whether the deaths could have been linked to carbon monoxide poisoning.
'No major trauma'
He said there was no suggestion the deaths were drug-related.
The house has been taped off by police and forensic investigators have been at the scene.
Police are also carrying out house-to-house inquiries in the local area.
Mr Jones said: "There is no obvious sign of major trauma and we await the outcome of the post-mortem to determine the cause of death."
He said that although their deaths were not being treated as suspicious at this stage, the level of resources usually committed to a murder inquiry had been allocated to the investigation.
West Midlands Police said the families of the couple were being supported.