The people of Liverpool have been urged to turn in the masked gunman who killed a nine-year-old girl as he chased his intended target into her home.
Merseyside Police Chief Constable Serena Kennedy said the "shocking" killing of Olivia Pratt-Korbel "crosses every single boundary" as the force appealed to the "criminal fraternity" in Liverpool for information.
The BBC reported two sources had come forward and given the same name to Merseyside Police as of Wednesday morning.
Olivia died on Monday night when a 35-year-old man, unknown to the family, ran into her terraced house in Kingsheath Avenue, in the Dovecot area of Liverpool, in an attempt to get away from a shooter, police said.
Her mother Cheryl Korbel, 46, was shot in the wrist as she tried to close the door on the gunman while Olivia stood behind her.
Ms Kennedy told a press conference on Tuesday: "This is a shocking and appalling attack which will reverberate around our communities, and I want to take this opportunity again to appeal to anybody who knows who was responsible for this attack to please come forward and give us those names.
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"We need to find all who are responsible for - not just the gunman, we need to find who supplied the weapon and who arranged this terrible incident."
The Liverpool Echo said the killing had "shocked the city" and made a passionate front page appeal for help, by telling its readers to "imagine the agony of Olivia's family as they are told she did not make it - murdered in her own home".
The newspaper stated: "Imagine knowing something about this unspeakable crime but keeping quiet because you would rather surrender our city to the thugs than to get justice for Olivia.
"If this is you, ask yourself ... whose side are you on?"
The man who had entered the family home suffered gunshot wounds to his upper body and, as Olivia lay dying, was picked up and taken to hospital by friends driving a dark-coloured Audi.
Ms Kennedy said: "We will not rest until those who are responsible are put behind bars. I can guarantee that no stone will be left unturned."
Olivia went to St Margaret Mary's Catholic Junior School in Huyton, where she was thought of as a kind-hearted, helpful and happy little girl, according to her headteacher Rebecca Wilkinson.
She said: "Olivia was a much-loved member of our school. She had a beautiful smile, a lovely sense of humour and a bubbly personality.
"She was kind-hearted and would go out of her way to help others."
Pupils of the school have visited the scene to put down teddies and flowers.
Family members have also placed down tributes. A handwritten note on one of the bouquets of flowers reads: "RIP baby girl , can't believe what has happened, feels like a nightmare. We will miss you so much our sassy queen. Love aunty Kim and uncle Tony."
Another reads: "Our Liv, our brown-eyed sassy pants. I will never find the words to describe the heartache we have all felt. I'll miss your outgoing, sassy little self and all the joy, laughs, smiles, love you brought to our lives. Love you forever and always."
Prime Minister Boris Johnson described the shooting as an "unimaginable tragedy" and promised that Merseyside Police would get "whatever they need to catch those responsible".
The killing happened exactly 15 years after 11-year-old Rhys Jones was fatally shot in Croxteth, Liverpool.
Detective Chief Superintendent Mark Kameen said Olivia had been at home with her two older siblings and mother when Ms Korbel opened the door after hearing gunshots fired outside.
He said a figure, wearing a black padded jacket, a black balaclava with a peak, dark trousers and black gloves, fired shots at two men walking along Kingsheath Avenue, causing both of them to run away.
One of the men ran towards the open door of Olivia's home and forced his way in.
Mr Kameen said: "As that was taking place the person with the gun has followed the male to this property.
"That person has also tried to force entry to the property and has managed, it would appear, to put their hand through the open door as Cheryl continued to try and close it.
"A shot has been fired which we believe has hit Cheryl, injuring her and then fatally wounding Olivia. Olivia at that time, we believe, was stood directly behind Cheryl."
Olivia was taken to Alder Hey Children's Hospital by officers, but later died.
The Audi used to transport the injured man has since been seized.
Mr Kameen said: "My appeal is to the person responsible for this horrendous attack on a nine-year-old schoolgirl to recognise the pain and anguish that this has caused to her family.
"I want that individual to hand themselves in."
Barbara Murray, Labour councillor for the Yew Tree ward in Liverpool where Olivia lived, repeated police calls for anyone with information to come forward.
She told BBC Breakfast: "Coming forward, you don't have to go to the police. (Liverpool) has got 90 councillors, I think any one of them would liaise with police on behalf of anybody who wants to give information.
"Really strongly, I would say people can use Crimestoppers. It's an anonymous phone call. You don't have to give information about yourself.
"When something like this happens, it's always the case, someone, somewhere will know who (the perpetrator) is."
The former chief constable of Greater Manchester Police has downplayed the likelihood of criminals turning in the killer.
Questioned on Sky News about whether criminal gangs would respond to an appeal from Merseyside Police, Sir Peter Fahy said: "It's very rare, really.
"This, at the end of the day, will be down to either a lucky piece of information from the public, really good analysis of mobile phone data.
"Sometimes another criminal gang will see to their advantage in a way to pass forward information which might lead to the disruption of another criminal gang.
"Often when criminal members suffer the most horrendous injuries they refuse to co-operate with the police or tell the police what's happened.
"Because they want to exact their own revenge or just out of fear."
