An emergency 999 call was made by a man from the address where four people were found dead but police failed to respond, it has emerged.
Officers discovered the bodies, including a father and two young girls, believed to be his daughters, more than an hour later at around 7.15am on Friday after forcing their way into the property in Allan Bedford Crescent, Costessey, near Norwich.
They had been alerted by a member of the public, who was concerned for the welfare of the people inside, shortly before 7am.
The Norfolk force has referred itself to the Independent Office of Police Conduct (IOPC) watchdog in relation to the 6am call "to which police resources were not deployed".
Police had already contacted the IOPC over the incident because officers attended the property on 14 December last year in relation to a missing person inquiry.
The 45-year-old man found dead at the address has been named as Bartlomiej Kuczynski.
Alongside him were two young girls and a 36-year-old woman, said to be part of the same family.
The Norfolk force had previously said all four were found with injuries.
Police said on Saturday night that they were not looking for anybody else in relation to their investigation.
Nina Crisan, whose son knew one of the girls, Jasmine, who lived at the house, described her as a "very, very sweet girl".
"She had very calm and cool energy, as my son would say," she told Sky News.
"She was just a lovely, lovely, lovely kid."
Ms Crisan also described the family as "very friendly" and said the incident had come as an "absolute shock" to local residents.
A boy who knew Jasmine, speaking anonymously to Sky News, said: "We grew up together. She never held a grudge, she was always kind and caring to everyone.
He added: "I never really thought what my last words to her would be."
Another said: "She used to play with my little sister in the park, she used to look out for her. It is really sad for the whole community."
A police cordon remains in place and forensic investigations continued on Saturday inside the property.
Detectives have also been carrying out house-to-house enquiries, speaking with witnesses and examining CCTV footage.
Detective Chief Inspector Chris Burgess, who is leading the enquiry, said: "We've had a heavy police presence in the area since yesterday morning and there's understandably concern in the local community. However, their help and support in the investigation has been unwavering and on behalf of everyone involved in this devastating incident, we're extremely grateful for that.
"Today, the focus of our enquiries remains at the address and we're examining local CCTV.
"While we're still piecing together what's happened, at this stage we remain satisfied that this is an isolated incident."
Post-mortem examinations are to be held in the coming days to establish the cause of their deaths.
Mr Burgess added: "We're aware of commentary on social media and in news media, however, I would emphasise that formal identification is yet to take place.
"As I said yesterday [Friday], we believe the man and two children lived at the address, and while the woman was staying there, she wasn't a permanent resident."
Police have appealed for anyone with information to contact them.
A cordon remains in place at the scene and detectives from Norfolk and Suffolk police forces are leading the investigation.
The Bishop of Norwich Graham Usher said local clergy were offering support in the communities they serve and would be opening churches over the weekend for those who wish to pay tribute.
A post on the Diocese of Norwich's Facebook page said: "Our thoughts and prayers are with all those impacted by the tragic news in Costessey."