A woman died from a stab wound and four children were hurt in a domestic incident in which a man also suffered serious injuries.
Officers were called to a home in Reinwood Road, Huddersfield, at 9.32pm yesterday following calls from the public, and arrested a man.
They found a 37-year-old woman with a serious stab wound and she was taken to hospital but died.
A nine-year-old boy was found outside the property with a stab injury to his arm. He was treated in hospital.
West Yorkshire Police said another nine-year-old boy, an 11-year-old boy and a six-month-old baby girl were also present and treated for minor injuries.
Senior Investigating Officer, Detective Chief Inspector Ady Taylor said: "A 39-year-old man was detained at the scene and is being treated in hospital today for serious injuries.
"We are not looking for anyone else in connection with this incident.
"Enquiries are ongoing to establish the full circumstances surrounding the incident and Neighbourhood Policing Team officers are patrolling the area, offering support and reassurance to local residents.
"I would ask any witnesses or anyone with information to contact Protective Services Crime on the non emergency number 101 or Crimestoppers in confidence on 0800 555 111."