The teenager who murdered Ava White told a police officer "shut up you nonce" when he was being questioned about the killing in an interview.
Ava, 12, was stabbed in the neck by a boy, 14, in a row over him filming her on Snapchat.
A trial heard how Boy A, who cannot be named for legal reasons, smirked and and laughed after the incident, then ran away and dumped his flick knife, designer jacket and mobile phone in a "cover up".
The killer, accused of showing a "callous disregard" for the schoolgirl, took selfies, got butter for crumpets and played the video game Call of Duty at a friend's house while Ava was dying in Liverpool city centre.
Liverpool Echo reports that when he was phoned by his mum and told that detectives were looking for him, he hung up then messaged her: "Gonna end up going the cells for nothing."
Boy A claimed he acted in self-defence, but a jury took just two hours and eight minutes to unanimously find him guilty of murder at Liverpool Crown Court. He started crying as members of Ava's family cheered, clapped and shouted "get in".
The youth was arrested at around 10.30pm on November 25 last year - just two hours after the murder.
The jury heard edited transcripts of five police interviews carried out in the days that followed.
Prosecutors outlined a series of lies he told police - including giving a "false alibi" and blaming another boy for the stabbing.
Liverpool Echo can now reveal his outburst in his first interview which the jury never heard.
Boy A refused to tell police where his phone was because he didn't want it taken off him; falsely claimed to have been at his friend's flat and refused to give the address, stating "it's not my business to tell you"; and lied about the coat he was wearing.
When asked what would happen if detectives found CCTV footage proving he had been in town, he replied "well you won't"; and when told police would find out his phone number, he replied: "I'm not bothered."
Asked "why do you think you're here?" he answered "I don't f***ing know". A detective said they believed he was involved in Ava's death and asked "are we wrong?" Boy A replied "you've asked me like a hundred times.... yeah, you are".
Following legal discussions, it was decided the jury would not hear that he then told an officer: "Shut up you nonce." He also referred to "smoking weed" in part of an interview not read to the jury.
When shown CCTV footage proving he was in town, Boy A admitted in a second interview he was present and claimed he had been "fronted" over filming a video of Ava "messing around" lying on the ground.
His solicitor said "will you stop it?" and the teenager said "I'm not even arsed".
He claimed two older boys who knew Ava's group of friends had threatened to stab one of the boys with him unless that boy deleted a video he had filmed.
Boy A said Ava's group had then talked about how they were going to "batter" him, but he still denied being there when she was stabbed.
Boy A said he left the coat he had been wearing in the garden of his nan's house, but didn't want to tell police where it was, because they would take it.
Detectives said he would get it back, to which he replied "what in like eight months when it won't fit?" It was later found in a wheelie bin.
In a third interview, he finally admitted being in School Lane and said Ava had tried to hit him, before saying "it just went all weird". He started crying and was given a break, before the interview resumed and he suggested another boy stabbed Ava.
Boy A suggested this innocent boy made him swap clothes after the killing and said he would pay him. When challenged over this lie, he became "agitated", stood up, shouted "move" and tried to leave the interview room.
In a fourth interview, he repeatedly told police "go on, blame it on me" and complained that he would be made to go into care. When told if he was responsible for the stabbing there was a "good chance" witnesses would pick him out in an identity parade, Boy A said: "We will see, won't we?"
It was only in a fifth interview he admitted lying about the other boy being responsible. Asked why he had lied, he replied: "No comment."
The boy was accompanied by an intermediary throughout the trial, which he attended over video link. He was given a fidget toy, which the jury was told could help him concentrate, due to his ADHD.
In his evidence, the teen was asked why he lied to police and said he thought he would "get away with it". He added: "I was scared I was going to go to jail."
He was also questioned why he hadn't agreed to give his phone to the police. He said: "Because they always take my phone. I have had a few phones took when I was in the police station."
In March, the boy's legal team contacted police to tell them the whereabouts of the knife. He was asked in court why he wanted police to have it and said: "Because I'm telling the truth and I didn't mean to do it."
Boy A, who admitted possessing an offensive weapon, was asked why he carried the knife with a 7.5cm long blade. His only explanation was "because I thought I was big".
Boy A will be sentenced on July 11, when he faces a life sentence.
Speaking after the case, senior crown prosecutor Nicola Wyn Williams, of Crown Prosecution Service (CPS) Mersey Cheshire's Complex Casework Unit, said: "This is a tragic case in which a 12-year-old child is dead and another child has been found guilty of her murder.
"The Crown Prosecution Service has worked hard to deal with the fact that the victim, defendant and many of the witnesses in this case were children.
"We have worked closely with the victim's family to help them to understand what was happening. We have provided a variety of special measures to support and protect the child witnesses and the trial process took into account the age of the defendant."
Ms Wyn Williams added: "At the heart of this case is a knife that was used to inflict a fatal wound on Ava White. Knife crime is taking a heavy toll on society and many of the victims are young. The message of the Crown Prosecution Service is - do not carry a knife.
"This case illustrates, amongst many other things, the dreadful cost of carrying a knife."
Clare Tripcony, head of CPS Mersey-Cheshire's Complex Casework Unit, said: "The Crown Prosecution Service pursued a charge of murder in this case.
"We argued that this defendant meant to do Ava White serious harm – he plunged a 7.5 cm knife into her neck. His actions were deliberate and the jury have agreed with that.
"Unfortunately, the success of this prosecution will not bring Ava White back. That is what her family have now to try and come to terms with in some way.
"The Crown Prosecution Service would like to extend our condolences to them and thank them for their courage and dignity throughout this dreadful case."
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