Isabelle Sanders, 51, and Norman Busby, 85, were in their home in a quiet cul-de-sac in Crookston, Glasgow, when they discovered a man inside the property.
Officers arrived at the house just before 2am and found the body of Ms Sanders and her partner seriously injured. Detectives would not reveal who called police.
Mr Busby, a retired policeman, was taken to the Southern General Hospital, where he is in a serious condition.
The couple both have children from previous relationships but it is believed they had lived together in Crookston for more than 20 years.
Mr Busby has two sons Donald, 52, of Thurso, and Norman, 58, of Glasgow, and a daughter Patricia. Last night Donald said: "I am travelling down from the north of Scotland to see my dad. I'm not sure how he is, I've only had a brief conversation with my sister. I don't know much about what happened."
Forensic officers spent hours combing the scene for clues yesterday in a bid to find out more about the killing.
A car that was parked in the driveway remained covered in blue tarpaulin until it was removed by officers shortly before 3pm.
Superintendent Brian McInulty, of Police Scotland, declined to go into details about how the vehicle was linked to the investigation.
Police issued a description of man reportedly seen at the house. He is a white male, aged 20 to 30, around 5ft 10in tall, of slim build with short dark hair.
He spoke with a local accent and was wearing jeans, a light blue top and blue canvas deck shoes.
Residents in the quiet area told of their shock.
Neighbour Robert Freckleton, 61, said: "My niece came out to go to her work and saw all the activity with the police. It is a pretty quiet street, but there has been one or two incidents recently. I had my car tried the other week by somebody trying to get into it.
"But we've never had anything like this. It is clearly being taken very seriously and my thoughts are with those involved."
Another resident said: "I'm absolutely shocked and devastated. I know that family well because I used to walk the dog with them.
"I've lived here for the last 13 years or so and we've never had anything like this before. It's a quiet area where folk keep themselves to themselves, we don't have any trouble. I just can't believe this has happened. This is such a tragedy."
Police said they were still trying to establish a motive for the killing. They were carrying out door-to-door inquiries and were checking CCTV footage as part of their investigation.
Superintendent McInulty said: "The circumstances of this incident are very unusual.
"The man has lost his partner and he is obviously upset and disturbed but he is working very well with our officers and we are supporting him while he is in hospital. We want to catch to the person responsible for this despicable crime and we are working tirelessly."
He added: "This is a very safe community. Patrols have been stepped up, and anyone with any concerns should approach officers."