Murdered by a stranger: Psychiatric patient who stabbed girl, 13, to death with a kitchen knife as she walked to a friend's sleepover is jailed for life Hannah Bonser, 26, found guilty of murder after just two-hour deliberation Psychiatric patient given minimum 22-year prison term for 'random attack' She attacked the teenager in a park in Doncaster with a kitchen knife she bought the same morning Teen's father describes killing as 'calculated and deliberate' Bonser has a troubled mental history and spent time in a Doncaster phsychiatric hospital in 2011 Three days before the attack Bonser told a friend she was going to kill her ex-boyfriend and ‘get anyone else who crossed her’, court heard By CHRIS PARSONS UPDATED: 15:50 GMT, 11 July 2012 Guilty: Hannah Bonser, pictured when she appeared at before magistrates earlier this year, killed Casey Kearney, 13, at a park in Doncaster A psychiatric patient who stabbed a teenage girl to death with a kitchen knife was jailed for life today after being convicted of murder. Hannah Bonser, who has a history of mental health problems, knifed 13-year-old Casey Kearney to death on Valentine's Day as the teenager walked through a park to a friend's sleepover. The 'happy, ordinary' youngster was in 'the wrong place at the wrong time' when she was set upon by Bonser, 26, in the 'random attack' in Elmfield Park, Doncaster, South Yorkshire. Her mother, Kerry Day, has described how it had only been a couple of months since she had started letting her daughter use the bus to go into town to meet friends. As the pair – who had never met – passed each other in a park, Bonser plunged a large kitchen knife into the schoolgirl’s chest before calmly walking off, her trial previously heard. Casey managed to ring 999 on her mobile phone and collapsed. She was helped by passers-by and taken to hospital, but died that evening. A jury at Sheffield Crown Court today rejected Bonser's claim that she was guilty only of manslaughter due to diminished responsibility, instead convicting her of murder. A judge who sentenced her to a minimum 22-year prison term said the killing was 'horrific'.Bonser, who was said to have a ‘troubled history’, had been treated in a psychiatric hospital four months earlier and had been involved with an organisation helping those with mental health problems. Much-loved: A friend described murdered Casey Lyanne-Kearney as a 'really lovely' girl with 'loads of friends' Schoolgirl: Casey in her uniform, left, and taking a picture of herself with her phone, right Murder scene: The forensics tent mark the place of the attacks beside a children's playground where two ten-year-olds were playing at the time Just three days before the attack she told a friend she was going to stab her ex-boyfriend to death because he made her have an abortion years ago and she was ‘going to get anyone else who crossed her in her life,’ Sheffield Crown Court was told. The court heard that on the day she was killed, Casey caught the no 55 bus towards Doncaster town centre, but missed the stop where she was planning to get off. She still used the same park entrance after walking back from the next bus stop but who knows whether, if she had got off at the right stop, she would have missed Bonser altogether. Sadness: Friends of tragic teenager Casey embrace at Elmfield Park in Doncaster where she was stabbed to death Attack: Casey, pictured at a family event holding a baby, managed to call police after she was stabbed As one lawyer said in court: 'She really, really was in the wrong place at the wrong time.' Sentencing her, Judge Mr Justice Cranston said: 'Why did Hannah Bonser do this to Casey - a young girl, a complete and wholly innocent stranger?' He said she suffered a 'horrific death'. Speaking outside court after the sentencing, Casey's father said her murder 'means we will never have the privilege of her in our lives again'. Mr Kearney said: 'We think the jury has come to the correct decision in convicting Bonser with the murder of Casey. 'There was never a doubt in our minds this was a calculated and deliberate act and therefore deserves the maximum sentence our judicial system allows. 'However, no sentence will bring Casey back to us and the severity of Bonser's actions mean we will never have the privilege of her in our lives again.' Bonser showed no emotion when the jury foreman returned the guilty verdict after two hours of deliberation. She sat in the dock looking ahead, as she has done throughout the week-long trial, surrounded by four prison officers. There were cries of 'Yes' from the public gallery which was packed with Casey's family. Her father, Anthony Kerney, appeared to wipe away tears. Many in the gallery were wearing orange ties and scarves - one of Casey's favourite colours - in memory of the teenager. Bonser will be sentenced later this afternoon. Bonser, of Cusworth House, Doncaster, had denied murder. She has a history of mental health problems and her defence team claimed she is a paranoid schizophrenic. Her barrister, David Fish QC, had asked the jury of seven women and three men to find his client guilty of manslaughter due to diminished responsibility or lack of intent. But prosecutors argued that she suffers from a personality disorder, not a psychosis, and was guilty of murder. The jury agreed. Violence: The injured teenager managed to stagger away after the attack in Elmfield Park, Doncaster and dialled 999 The judge told the court that Bonser will be jailed for life and the minimum term will be fixed this afternoon. In a statement to police, Mrs Day recalled her daughter leaving the family home for the last time to go to the sleepover. She said: 'We both said ‘I love you' and I told her to watch the road next to Lucia's. I told her to use the crossing.' After Casey's death, Mrs Day and Mr Kearney paid tribute to their daughter and said: 'She was the most beautiful, intelligent and bright young girl with her whole life ahead of her.' In the weeks which followed, the community of Rossington pulled together in memory of Casey. More than 200 people took part in two charity walks and a concert to raise money for her family.