This case has shone a light on organised crime, drugs and guns on Liverpool, and BBC Panorama has been investigating how they have taken hold in the city.
Reporter Bronagh Munro has spoken to people who used extreme violence to sell drugs, including former gang leader Darren Gee.
"Without violence, you can't deal drugs," said Gee, adding that his gang had been involved in gun crime, knife crime and kidnapping.
He was jailed in 2006 for conspiring to murder someone in a revenge attack.
But he got the wrong man killed.
Watch as he is asked how he feels about the crime now.
Video caption: 'Without violence, you can't deal drugs''Without violence, you can't deal drugs'
Thanks for joining us
If you want more on this story, you'll find plenty of video clips at the top of this page, and you can read our article pulling the story together here.
Police and family thanked a key witness for coming forward after Olivia's murder - you can read about her brave decision to do so here.
And there's more on how police are trying to tackle gun crime here.
Today's contributors were Aoife Walsh, Oliver Slow, Lauren Hirst, Krystyna Gajda and Jack Burgess alongside our North of England correspondent Judith Moritz, who was in court.
WATCH: 'Cashman won't be out for a very long time' - police
Detective Superintendent Mark Baker, who led the murder investigation, says he hopes the sentence handed down today will act as a deterrent for anyone who thinks of picking up a gun.
In a statement to reporters, he also thanks the key witness for coming forward and helping to bring justice.
Video content
Video caption: 'Cashman won't be out for a very long time' - police'Cashman won't be out for a very long time' - police
Cashman returned to prison
A prison van believed to be carrying Thomas Cashman has left Manchester Crown Court after he was sentenced.
Cashman refused to appear at today's hearing, telling his lawyer that the trial had "turned into a circus".
PA MediaCopyright: PA Media
WATCH: Happy but not celebrating - Olivia's father
Louise Pratt also gave a statement outside Manchester Crown Court - she was speaking on behalf of Olivia's father, John.
He thanked the jury for their verdict, as well as the key witness who "showed courage and stood up for Olivia".
Video content
Video caption: Happy but not celebrating - Olivia's fatherHappy but not celebrating - Olivia's father
WATCH: 'Everyone adored her'
Outside court, Olivia's mother Cheryl Korbel paid tribute to her daughter, who she described as "a social butterfly" and "such a caring little girl".
"Justice has prevailed, and I cannot begin to express our relief," she said, thanking all those involved in the investigation, including the police and witnesses.
"My family and I have already started out life sentence, having to spend the rest of our lives without Olivia," she said.
Video content
Video caption: 'Everyone adored her' - mother's tribute to Olivia'Everyone adored her' - mother's tribute to Olivia
Watch the moment Cashman is jailed for 42 years
As the judge delivered her sentence, she said there were no mitigating factors in Thomas Cashman's history, nor had he shown any remorse.
Watch the moment she sentenced Cashman, who refused to enter the courtroom:
Video content
Video caption: Olivia Pratt-Korbel murder: Thomas Cashman has shown 'no remorse'Olivia Pratt-Korbel murder: Thomas Cashman has shown 'no remorse'
Cashman won't be released until he's an old man - police
BBCCopyright: BBC
We're now hearing from Merseyside Police's Detective Superintendent Mark Baker, who led the murder investigation.
He says Thomas Cashman "won't be out for a very long time and he'll be an old man he is".
He also thanked the key witness for coming forward and helping to bring justice - "she did the correct thing and she should be proud of herself".
He adds that the courage and bravery shown by Olivia's family is in direct contrast to the "cowardice" shown by Cashman.
He ends by saying: "I hope the sentence will act as a deterrent for anyone who thinks of picking up a gun and putting our community at risk through their reckless behaviour."
We have already started our life sentence - Olivia's mother
Olivia's mother Cheryl Korbel says she welcomes Thomas Cashman's 42-year sentence, but says their family has "already started our life sentence, having to spend the rest of our lives without Olivia".
She finished by thanking the public for the "overwhelming support" the family has received, and asked for "time and space to process what has happened to us".
'We now draw a line under seven months of torment'
Olivia's mum Cheryl tells reporters Olivia was the light of her family's lives, "a sassy, chatty girl" who never ran out of energy.
"Everyone adored her," she adds.
She says all the promise for Olivia's future was cruelly taken away and everywhere they go is a reminder that she is not there with them.
"We can now draw a line under seven months of torment we have had to endure at the hands of Cashman," she says.
She was a chatty girl, a character, my baby - Olivia's mother
PA MediaCopyright: PA Media
Olivia's mum, Cheryl Korbel, is now speaking outside court, wearing a jumper showing Olivia's face.
She describes Olivia as a "beautiful girl" and a "social butterfly".
"She was a chatty girl, a character and my baby," she says, adding that she had amazing qualities and knew what she wanted in life.
'We hope Cashman is haunted by Olivia's death'
More now from Olivia's family, who are speaking outside Manchester Crown Court.
Louise Pratt, Olivia's aunt, says her niece "died a scared nine-year-old" and the family hopes Cashman "is haunted by this knowledge for the rest of his life".
We are happy, but not celebrating - Olivia's father
ReutersCopyright: Reuters
Olivia's aunt is reading a statement outside court on behalf of the nine-year-old's father, John Pratt.
He says he is happy with the outcome, but he will not be celebrating as "nothing will fill the gap in our lives" caused by Olivia's killing.He thanked the jury for the verdict, as well as the key witness who "showed courage and stood up for Olivia".
Family expected outside court soon
We're still waiting for reaction to today's sentencing outside Manchester Crown Court. We're expecting to hear from Olivia's family - both from her mum and dad's side - as well as from police who were involved in the investigation.
We'll bring you all the key lines as we get them.
Watch the moment Olivia's murderer was arrested
As we wait for family and police outside court, let's revisit the bodycam footage showing the moment Thomas Cashman was arrested on suspicion of murder in September last year.
A heads up, there's some mild swearing in the video.
Video content
Video caption: Thomas Cashman: Moment Olivia murderer is arrestedThomas Cashman: Moment Olivia murderer is arrested
What Cashman was sentenced for
A few moments ago, Thomas Cashman was handed a minimum of 42 years in prison over the murder of nine-year-old Olivia Pratt-Korbel.
The judge sentenced him on five counts:
Count one: For the murder of Olivia - the sentence will be life imprisonment. The minimum term will be 42 years
Count two: The attempted murder of Joseph Nee, the intended target of the attack. The sentence is life imprisonment with a minimum term of 22 years
Count three: For wounding Olivia's mother Cheryl Korbel, the sentence is 10 years imprisonment
Count four and five: The firearms offences - the sentence is 18 years imprisonment.
All sentences are to be served concurrently, which means the shortest period Cashman can serve before he can be considered for early release is 42 years, less the 182 days he served on remand.
Family file out of courtroom as hearing ends
The court hearing has now ended.
We're expecting to hear reaction from Olivia's family and the police outside the court shortly - we'll bring you all the key moments.
Remember, you can watch along by pressing play at the top of the page.
Gasps from public gallery
Judith Moritz
North of England correspondent in court
There were gasps from the public gallery as the sentence was announced.
Olivia's mum Cheryl and other relatives were crying, while some police officers were also tearful.
Outside the courtroom, Olivia's mum and aunties are hugging and crying.
BreakingCashman sentenced to minimum of 42 years in prison
Police handoutCopyright: Police handout
Thomas Cashman has been sentenced to a minimum of 42 years in prison for the murder of Olivia Pratt-Korbel.
Olivia's murder was chilling - judge
PA MediaCopyright: PA Media
The judge says Cheryl should have been able to reassure Olivia and "tuck her back into bed".
"What happened instead was chilling," she adds.
She says she does not agree that Olivia's age made her more vulnerable to being killed in these circumstances.
"The real gravity is that a young child was shot and killed in her own home", Mrs Justice Yip tells the court.
She says had the defendant planned to kill Olivia he would have been facing a whole life order, which would mean he would have to spend the rest of his life in prison.
Live Reporting
Edited by Dulcie Lee
All times stated are UK
Video caption: 'Without violence, you can't deal drugs''Without violence, you can't deal drugs' Video caption: 'Cashman won't be out for a very long time' - police'Cashman won't be out for a very long time' - police 
PA MediaCopyright: PA Media Video caption: Happy but not celebrating - Olivia's fatherHappy but not celebrating - Olivia's father Video caption: 'Everyone adored her' - mother's tribute to Olivia'Everyone adored her' - mother's tribute to Olivia Video caption: Olivia Pratt-Korbel murder: Thomas Cashman has shown 'no remorse'Olivia Pratt-Korbel murder: Thomas Cashman has shown 'no remorse' 
BBCCopyright: BBC 
PA MediaCopyright: PA Media 
ReutersCopyright: Reuters Video caption: Thomas Cashman: Moment Olivia murderer is arrestedThomas Cashman: Moment Olivia murderer is arrested - Count one: For the murder of Olivia - the sentence will be life imprisonment. The minimum term will be 42 years
- Count two: The attempted murder of Joseph Nee, the intended target of the attack. The sentence is life imprisonment with a minimum term of 22 years
- Count three: For wounding Olivia's mother Cheryl Korbel, the sentence is 10 years imprisonment
- Count four and five: The firearms offences - the sentence is 18 years imprisonment.
Police handoutCopyright: Police handout 
PA MediaCopyright: PA Media
Latest PostThere's more to watch on iPlayer
This case has shone a light on organised crime, drugs and guns on Liverpool, and BBC Panorama has been investigating how they have taken hold in the city.
Reporter Bronagh Munro has spoken to people who used extreme violence to sell drugs, including former gang leader Darren Gee.
"Without violence, you can't deal drugs," said Gee, adding that his gang had been involved in gun crime, knife crime and kidnapping.
He was jailed in 2006 for conspiring to murder someone in a revenge attack.
But he got the wrong man killed.
Watch as he is asked how he feels about the crime now.
You can watch the full Panorama film - The Drug Wars that Killed Olivia – on BBC iplayer now and on BBC One at 20:00 BST (UK).
Video content
Thanks for joining us
If you want more on this story, you'll find plenty of video clips at the top of this page, and you can read our article pulling the story together here.
Police and family thanked a key witness for coming forward after Olivia's murder - you can read about her brave decision to do so here.
And there's more on how police are trying to tackle gun crime here.
Today's contributors were Aoife Walsh, Oliver Slow, Lauren Hirst, Krystyna Gajda and Jack Burgess alongside our North of England correspondent Judith Moritz, who was in court.
WATCH: 'Cashman won't be out for a very long time' - police
Detective Superintendent Mark Baker, who led the murder investigation, says he hopes the sentence handed down today will act as a deterrent for anyone who thinks of picking up a gun.
In a statement to reporters, he also thanks the key witness for coming forward and helping to bring justice.
Video content
Cashman returned to prison
A prison van believed to be carrying Thomas Cashman has left Manchester Crown Court after he was sentenced.
Cashman refused to appear at today's hearing, telling his lawyer that the trial had "turned into a circus".
WATCH: Happy but not celebrating - Olivia's father
Louise Pratt also gave a statement outside Manchester Crown Court - she was speaking on behalf of Olivia's father, John.
He thanked the jury for their verdict, as well as the key witness who "showed courage and stood up for Olivia".
Video content
WATCH: 'Everyone adored her'
Outside court, Olivia's mother Cheryl Korbel paid tribute to her daughter, who she described as "a social butterfly" and "such a caring little girl".
"Justice has prevailed, and I cannot begin to express our relief," she said, thanking all those involved in the investigation, including the police and witnesses.
"My family and I have already started out life sentence, having to spend the rest of our lives without Olivia," she said.
Video content
Watch the moment Cashman is jailed for 42 years
As the judge delivered her sentence, she said there were no mitigating factors in Thomas Cashman's history, nor had he shown any remorse.
Watch the moment she sentenced Cashman, who refused to enter the courtroom:
Video content
Cashman won't be released until he's an old man - police
We're now hearing from Merseyside Police's Detective Superintendent Mark Baker, who led the murder investigation.
He says Thomas Cashman "won't be out for a very long time and he'll be an old man he is".
He also thanked the key witness for coming forward and helping to bring justice - "she did the correct thing and she should be proud of herself".
He adds that the courage and bravery shown by Olivia's family is in direct contrast to the "cowardice" shown by Cashman.
He ends by saying: "I hope the sentence will act as a deterrent for anyone who thinks of picking up a gun and putting our community at risk through their reckless behaviour."
We have already started our life sentence - Olivia's mother
Olivia's mother Cheryl Korbel says she welcomes Thomas Cashman's 42-year sentence, but says their family has "already started our life sentence, having to spend the rest of our lives without Olivia".
She finished by thanking the public for the "overwhelming support" the family has received, and asked for "time and space to process what has happened to us".
'We now draw a line under seven months of torment'
Olivia's mum Cheryl tells reporters Olivia was the light of her family's lives, "a sassy, chatty girl" who never ran out of energy.
"Everyone adored her," she adds.
She says all the promise for Olivia's future was cruelly taken away and everywhere they go is a reminder that she is not there with them.
"We can now draw a line under seven months of torment we have had to endure at the hands of Cashman," she says.
She was a chatty girl, a character, my baby - Olivia's mother
Olivia's mum, Cheryl Korbel, is now speaking outside court, wearing a jumper showing Olivia's face.
She describes Olivia as a "beautiful girl" and a "social butterfly".
"She was a chatty girl, a character and my baby," she says, adding that she had amazing qualities and knew what she wanted in life.
'We hope Cashman is haunted by Olivia's death'
More now from Olivia's family, who are speaking outside Manchester Crown Court.
Louise Pratt, Olivia's aunt, says her niece "died a scared nine-year-old" and the family hopes Cashman "is haunted by this knowledge for the rest of his life".
We are happy, but not celebrating - Olivia's father
Olivia's aunt is reading a statement outside court on behalf of the nine-year-old's father, John Pratt.
He says he is happy with the outcome, but he will not be celebrating as "nothing will fill the gap in our lives" caused by Olivia's killing.He thanked the jury for the verdict, as well as the key witness who "showed courage and stood up for Olivia".
Family expected outside court soon
We're still waiting for reaction to today's sentencing outside Manchester Crown Court. We're expecting to hear from Olivia's family - both from her mum and dad's side - as well as from police who were involved in the investigation.
We'll bring you all the key lines as we get them.
Watch the moment Olivia's murderer was arrested
As we wait for family and police outside court, let's revisit the bodycam footage showing the moment Thomas Cashman was arrested on suspicion of murder in September last year.
A heads up, there's some mild swearing in the video.
Video content
What Cashman was sentenced for
A few moments ago, Thomas Cashman was handed a minimum of 42 years in prison over the murder of nine-year-old Olivia Pratt-Korbel.
The judge sentenced him on five counts:
All sentences are to be served concurrently, which means the shortest period Cashman can serve before he can be considered for early release is 42 years, less the 182 days he served on remand.
Family file out of courtroom as hearing ends
The court hearing has now ended.
We're expecting to hear reaction from Olivia's family and the police outside the court shortly - we'll bring you all the key moments.
Remember, you can watch along by pressing play at the top of the page.
Gasps from public gallery
Judith Moritz
North of England correspondent in court
There were gasps from the public gallery as the sentence was announced.
Olivia's mum Cheryl and other relatives were crying, while some police officers were also tearful.
Outside the courtroom, Olivia's mum and aunties are hugging and crying.
BreakingCashman sentenced to minimum of 42 years in prison
Thomas Cashman has been sentenced to a minimum of 42 years in prison for the murder of Olivia Pratt-Korbel.
Olivia's murder was chilling - judge
The judge says Cheryl should have been able to reassure Olivia and "tuck her back into bed".
"What happened instead was chilling," she adds.
She says she does not agree that Olivia's age made her more vulnerable to being killed in these circumstances.
"The real gravity is that a young child was shot and killed in her own home", Mrs Justice Yip tells the court.
She says had the defendant planned to kill Olivia he would have been facing a whole life order, which would mean he would have to spend the rest of his life in prison.