All three Vishnu Mathur, 78, Shashi Mathur, 75, and Khushbu, 20 had their throats slit when the police reached the first floor flat in Basant Gaon near Vasant Vihar.
"A police team found them in a pool of blood. All three had been attacked with sharp weapons," Deputy Commissioner of Police Devender Arya said.
Another domestic help of the couple revealed that when she came in the morning, she found the doors bolted from the outside. When she entered the house, she found the couple and their help sprawled on the floor, Arya said.
The Mathurs were on their bed while the maid lay dead in the drawing room.
"Our investigation show the killer or killers made a friendly entry and executed the crime early on Sunday. The house was not ransacked as the motive was not robbery. We are probing the case from all possible angles," the officer added.
Vishnu Mathur and Shashi Mathur settled down in Basant Gaon some 18 years ago after the man retired from the Central Government Health Scheme (CGHS). His wife worked earlier in the New Delhi Municipal Council (NDMC).
"Due to a fracture in a leg, Shashi Mathur could not walk for the last two years. For her nursing, Khushbu of Jharkhand, was hired by the couple through a placement agency," another police officer probing the case told .
Khushbu had started working for the Mathurs only six months ago, another maid said.
Shashi Mathur was reportedly very religious and often spent her day in a nearby temple inside the housing society until she suffered the fracture. The Mathurs' only son had passed away 35 years ago in an accident.
They had a daughter, Anita, who stayed in Greater Kailash in south Delhi and took her mother for medical treatment.
"We are suspecting acquaintances who was friendly with the family, made a friendly entry and killed them. Or the accused was known to Khushbu. A suspicious biker was caught in CCTV footage. We are trying to identify him," the officer said.
( With inputs from IANS )