Police did not attend a 999 call in which a man expressed concern for his mental state, more than an hour before he and three others were found dead in a house in Costessey near Norwich following a subsequent call from a dog walker.
Norfolk Police has provisionally identified the man, woman and two young girls who died as Bartlomiej Kuczynski, 45, Kanticha Sukpengpanao, 36, Jasmin Kuczynska, 12, and nine-year-old Natasha Kuczynska.
The bodies of the family members were discovered at an address in Allan Bedford Crescent at around 7.15am on Friday.
A man had called 999 from the property at around 6am that day, but police were not dispatched.
The force has referred itself to the Independent Office for Police Conduct (IOPC), and the watchdog said it has started an independent investigation into the contact with the man prior to the deaths.
IOPC regional director Charmaine Arbouin said: “Norfolk Constabulary established that the man had made a 999 call to the force shortly before 6am on January 19.
“During that call the man expressed concerns for his own mental state saying he was confused.
“He was advised to seek medical advice and police did not attend.
“At around 7am the force received a call from a dog walker concerned for the welfare of those within the home.
“Officers attended shortly after and, tragically, found the man, a woman and two children dead.
“Following a mandatory referral from the force we have decided to investigate and will be examining if the force’s handling of the contact they had with the man was appropriate and in line with force policy, training and procedures.”
The IOPC said it would be making contact with the families of the deceased to explain its role.
Norfolk Police has also referred itself to the IOPC in relation to previous contact at the address on December 14 which was in relation to a missing person inquiry.
On Monday, Chief Constable Paul Sanford asked His Majesty’s Inspectorate of Constabulary and Fire and Rescue Services, which is already carrying out a routine inspection of Norfolk Police, to conduct a “deeper review” into his force.
Police are not looking for anyone else in connection with the deaths.
Post-mortem examinations carried out on Sunday found the man died of a stab wound to the neck, while the woman died of a number of stab wounds to the neck.
The bodies of the two girls will be examined on Wednesday.
Detectives said the man and two children lived at the address, but the woman was visiting and lived elsewhere.
All four were found with injuries.