Police have admitted they did not take action after receiving a report of dog breeding at the Liverpool house where a four-year-old boy was mauled to death.
John Paul Massey was killed by a dog at the property in Ash Grove, Wavertree, just after midnight.
His grandmother, Helen Foulkes, 63, was injured while trying to save the child and has been treated in hospital.
Merseyside Police said it had a report of dog breeding in February and was investigating why no action was taken.
It said the lack of action was "contrary to policy".
The animal which killed the boy was shot by armed officers in the front garden of the house.
An investigation is continuing to establish whether it was a breed deemed illegal under the Dangerous Dogs Act.
Residents: 'These dogs should not be allowed to be on the streets'
Ch Supt Steve Ashley, of Merseyside Police, said: "In February this year a housing officer rang Merseyside Police to say they had received complaints of dog breeding at the address.
"At that time the police said it wasn't a police matter and no action was taken by Merseyside Police.
"That is contrary to our policies and an investigation is now launched into exactly what happened in February."
Ch Supt Ashley said an officer who is an expert on dangerous dogs should have spoken to the person who was concerned and, if necessary, visited the house to examine the dogs.
Police said Ms Foulkes intervened when the dog attacked John Paul and managed to get it out of the house through the front door.
Paramedics went in through the back of the property to treat John Paul, but he died from his injuries.
John Paul's 13-year-old brother was also in the house and was not injured, but police said he had been left "deeply traumatised" by the attack.
Ms Foulkes has been released from hospital after treatment for dog bites to her arms, body and legs.
Residents spoke of their shock at the death but complained there had been a problem with dangerous dogs in the area.
Forensic officers examined the scene of the dog attack
Gillian Watson, 46, who heard the gunshot that destroyed the dog, said: "It's such a terrible thing to happen to a family.
"There are lots of dangerous-type dogs around here. You always see young lads with pit-bull dogs roaming around.
"I have a dog myself and when I take him for a walk sometimes it's quite terrifying because you think your dog is going to be attacked."
Michael Hennessey, head teacher at St Clare's RC Primary School, where John Paul was in the nursery class, said he was a popular pupil.
"He was a friendly, cheerful little boy who was bright and enthusiastic in everything he did.
"He will be very sadly missed, and the thoughts of everyone at the school are with his family."