The man investigators want to question over the stabbing of Emma Mansell and her children used to work for West Yorkshire Police.
Mansell was found stabbed to death at a home in the Lindley area of Huddersfield on Sunday.
Her children - Freddie, 11, and Oscar, nine - also sustained stab wounds to their arms, hands and bodies and are in hospital with serious but non-life threatening injuries.
A six-month-old baby girl was also present and had been treated for minor injuries.
Mansell's boyfriend Jonathan Sutton is under police guard at a hospital where he is reportedly undergoing treatment for stab wounds to his neck and throat.
According to reports, the 39-year-old man is alleged to have attempted to slit his own throat in the wake of the domestic incident.
It has also emerged that Sutton served in West Yorkshire Police force between May 2003 and November 2004.
Because he has been having surgery, detectives had last night been unable to formally arrest him on suspicion of murder.
'Covered in blood'
A family friend told The Mirror they found son Freddie shaking on the pavement outside the house, which had blood stains on a bedroom window.
She said: "He just kept saying 'I think he has killed my mum and my sister.' He was very distressed. Freddie was bleeding heavily. He was literally covered in blood. It was all over his face."
A West Yorkshire Police statement said: "The 39-year-old man who was detained also remains in hospital with non life threatening 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."