A father was being questioned by police after his partner and their eight-year-old daughter were found murdered at the family home.
John Miller, 36, a plasterer, also allegedly attacked his four-year-old daughter before leaving her with the bodies of her mother and sister, and driving to a nearby beauty spot to take his own life.
He was arrested after behaving suspiciously and apparently assaulting an ambulance crew trying to help him.
Officers then raced to the detached family home and found the bodies of his partner Sarah Laycock, 31, a primary schoolteacher, and their eldest daughter Abigail.
The youngest daughter was also found injured and was taken to hospital for treatment.
Last night, officers confirmed they were questioning the father-of-two on suspicion of murder and attempted murder.
No one else was being sought in connection with the crime, they added.
Details of the double murder emerged after police were called to a secluded car park in West Yorkshire, at 4pm on Saturday when a visibly distraught Mr Miller assaulted a paramedic who had come to his assistance.
They arrested Mr Miller and then went to the family home six miles away and found the bodies of Miss Laycock and their daughter Abigail.
They also found the youngest daughter, who has yet to be named, suffering minor injuries and shock and she was taken to Leeds General Infirmary for treatment but later discharged.
Last night neighbours reacted with disbelief to the incident and said that the family had always appeared happy.
Paula Clarke said: "They were just a lovely family. He worked hard and they all seemed very happy.
"The girls had got new bikes for Christmas.
"It's such an awful thing to happen, not something you would expect around here."
"Mr Miller was a keen trials motorbike rider and had a number of machines."
Another resident said: "They were never any trouble. He would park his van on the street and kept his motorbikes in the back garden.
"His girlfriend was pleasant and always had a smile. "It is horrific what has happened and I can't image what has gone wrong."
But a regular at the local Garforth Working Men's Club said work had dried up for Mr Miller and he had been spending more and more time in the gym.
The local man in his 50s, who did not wish to be named, said: "John had been getting less work so he'd been spending a lot of time at the gym bulking up."
"A four-year-old girl was also found with minor injuries. She was taken to Leeds General Infirmary and later discharged.
Only a month ago the West Yorkshire force attended a similar incident in Pudsey, after a kitchen designer Richard Smith, 37, murdered his wife Clair, 36, and their sons Ben, nine, and 13-month-old Aaron, before taking his own life.