Police Scotland said they received a report of a sudden death at a home at Raeburn Rigg, Livingston, late on Sunday evening.
Officers who had to force their way into the property in a quiet residential area of the town found a man and a woman dead inside.
They have been named locally as couple Denis and Mary Fell, both aged 73. Shocked Neighbours said Mrs Fell was a wheelchair user and had a dedicated disabled parking spot outside their property.
Police Scotland refused to comment on any possible motive for the deaths or what the 19-year-old detainee was being held for, fuelling speculation among local residents about the cause of the couple’s death.
In a brief statement, a spokeswoman for the force said: "Officers were made aware around 11.40pm on Sunday, December 26, of a report of a sudden death in the Raeburn Rigg area of Livingston.
"A man and a woman were found dead inside a property.
"A 19-year-old man has been arrested in connection with the incident and inquiries are ongoing to establish the full circumstances and are at an early stage."
Neighbours said they saw police arrive at the scene shortly before midnight and heard the doors being forced open.
A forensic medical examiner was called to the property and declared Mr and Mrs Fell dead at the scene. Shortly afterwards, the 19-year-old suspect was led away to a police vehicle and driven to a nearby station where he spent the day being questioned by detectives. It is expected the case will be handed over to the murder squad, now known as the Major Investigation Team.
Neighbours watched throughout Monday as forensic crime scene examiners studied the house for clues, taking photograph and removing bags of ‘evidence’. The area around the Fell’s home was taped off and a ‘Police Scotland’ temporary structure was erected to prevent anyone from seeing into the house.
One described Mr and Mrs Fell as a ‘very caring couple' and said their deaths had left the area in ‘a state of complete shock’.
She added: “They really were devoted to one another.
"You’d see Mr Fell pushing his wife out to the car in her chair and being very caring around her. They were both very caring, friendly and modest people.
"This has shocked everybody living here. You hear of these things happening but never thing it will be right on your own doorstep.”
Another neighbour, who said he had known Mr Fell for more than a decade, said: “Denis was a really nice guy. He was pretty humble and meek, the sort who would never cause any problem for anyone. God only knows what went on inside their house on Boxing Day. We know who the police took away but would rather not say their name publicly out of respect for Denis and Mary. They at least deserve that.”