By Chris Brooke and Anthony Bond UPDATED: 02:57 EST, 12 July 2012 Guilty: Hannah Bonser, pictured when she appeared at before magistrates earlier this year, killed Casey Kearney, 13, at a park in Doncaster An investigation is underway into why a woman with mental health problems who begged to be admitted to a hospital was turned away before going on to kill a schoolgirl. Hannah Bonser was jailed for at least 22 years yesterday after stabbing Casey Kearney, 13, to death. The 26-year-old had suffered years of mental problems caused by cannabis use and claimed she heard voices in her head telling her to kill.
@highlight
Hannah Bonser, 26, found guilty of murder after just two-hour deliberation
@highlight
Psychiatric patient given minimum 22-year prison term for 'random attack'
@highlight
She attacked the teenager in a park in Doncaster with a kitchen knife she bought the same morning
@highlight
Teen's father describes killing as 'calculated and deliberate'
@highlight
Bonser has a troubled mental history and spent time in a Doncaster psychiatric hospital in 2011
@highlight
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
Schoolgirl: @placeholder in her uniform, left, and taking a picture of herself with her phone, right