The dog that mauled a four-year-old boy to death in Liverpool was an illegal breed, police have confirmed.
John Paul Massey died from head and neck injuries in the attack at his grandmother's home in Wavertree.
Merseyside Police said the dog was a "pitbull terrier-type", a breed banned under the Dangerous Dogs Act. A criminal investigation has begun.
The force has apologised after earlier reports of dog breeding at the Ash Grove property were not followed up.
In February, a housing officer told police residents had complained about suspected dog breeding but were told it was not a police matter.
The animal also savaged the child's grandmother, Helen Foulkes, as she tried to protect the boy she was babysitting in the early hours of Monday.
John Paul's grandmother was hurt trying to stop the attack
The dog was shot dead by armed officers after Ms Foulkes managed to get it out of the house.
The 63-year-old suffered dog bites to her legs and body, but has since been discharged from hospital.
Deputy Chief Constable Patricia Gallan said establishing the breed of the dog had been "a complex issue".
"It has taken some time to complete the examinations and post-mortem before we have been able to confirm that it is an illegal type of dog," she said.
"We are conducting a criminal investigation."
The Independent Police Complaints Commission (IPCC) has confirmed it is investigating why the dog breeding reports were not followed up.
Deputy Chief Constable Gallan said: "On behalf of Merseyside Police, I am deeply concerned and very disappointed that our policy was not followed in February when we received a call alerting us to the possible breeding of pitbull terriers at the house where the attack took place.
"This is clearly unacceptable and we are sorry that Merseyside Police did not take the appropriate course of action at that time."
Naseem Malik, IPCC commissioner for the North West, said John Paul's death was "a terrible tragedy".
John Paul's mother left a message to her son on flowers outside the house
"My heartfelt sympathies go out to John Paul's family," she said.
"I fully appreciate the concern felt on Merseyside and around the country about the issue of dangerous dogs. We need to determine exactly what went wrong in this case to ensure lessons can be learned for the future."
Earlier, John Paul's mother, Angela, left a floral tribute outside the house.
Her message read: "My beautiful, beautiful boy, an angel who will never be forgotten."
"I will love you forever and a day. I miss you, baby. Lotsa love, Mummy xxx."
The Dangerous Dogs Act 1991 bans the breeding, sale or exchange of four kinds of dogs: pit bull terriers, Japanese tosas, the dogo Argentinos and the fila brasileiros.
Cross-breeds of these are also covered by the law. Other dogs that appear bred for fighting are also banned.
A dog classed as dangerously out of control in a public place can be destroyed and the owner fined or jailed for up to six months.
Owners can be imprisoned for a maximum of two years if their dog injures someone.
Deputy Chief Constable Patricia Gallan said a criminal investigation is under way
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