A woman fatally stabbed a 13-year-old girl in a random attack in a park just months after spending time in a psychiatric hospital, a court heard today.

Hannah Bonser, 26, is accused of murdering Casey Kearney in Elmfield Park, Doncaster, South Yorkshire, on Valentine's Day.

Bonser told a friend she planned to "get" anyone who crossed her and bought two knives before stabbing Casey, Sheffield Crown Court heard.

She may have prepared for the attack by stabbing a door in her flat, the jury was told.

Graham Reeds QC, opening the case for the prosecution, said Bonser had a "troubled history" and had spent a short time in a psychiatric hospital in October 2011.

She was in regular contact with outreach workers at Rethink - a service assisting those with mental health difficulties in Doncaster.

Mr Reeds described Bonser, of Cusworth House, Doncaster, as having few friends and "kept herself to herself".

But one friend described her behaviour as being "odd" after Bonser told her of plans to attack her ex-boyfriend just three days before Casey's death.

Mr Reeds said: "On 11 February, she went to see one of her friends, a woman called Hayley Spouse, and told her that she was going to stab her ex-boyfriend to death because he had made her have an abortion years ago and she hadn't wanted to do it, and then she was going to get anyone else who crossed her in her life."

On the morning of the stabbing, Bonser bought two kitchen knives before later going to Elmfield Park, the court heard.

Mr Reeds said Casey, from Rossington, near Doncaster, was walking through the park on her way to a friend's house when Bonser stabbed her in the abdomen without warning, causing "irreversible and unsurvivable" damage.

He said: "As they approached each other, the defendant pulled out the larger of her two knives.

"As she came alongside Casey, the defendant stabbed her once in the abdomen, causing such severe internal injuries that Casey later died."

Mr Reeds added: "The attack was sudden and caught Casey completely unawares.

"There was no argument, no shouting, there was no warning that anything was about to happen."

After the attack, Bonser carried on walking and left the park, the court heard.

Casey collapsed on the path, bleeding internally and losing consciousness, but managed to call 999 on her mobile phone.

"All she could say was that she had been stabbed," Mr Reeds said.

Mr Reeds said after the attack Bonser went to the Rethink offices and told an outreach worker she had killed somebody.

He said the defendant then produced the two knives. One had a 6.3in (16cm) blade and had Casey's blood on it.

Bonser was arrested but later declined to answer police questions, Mr Reeds said.

The court heard that, in a police interview, Bonser's solicitor said: "She's hearing voices telling her to do things and not to do things.

"It's a bit unclear what the situation is but they are obviously troubling her.

"It's something that has been with her for some time and she has decided she does not want to answer questions about this particular incident."

Following her arrest, police searched Bonser's flat and found marks and cuts on a door, which had been made with a knife in a "forcible upward movement", the court heard.

Mr Reeds said the marks were made at a similar height to the stab wound inflicted on Casey.

"Although there is no means of ascertaining when the cuts were made in the door, they raise legitimate questions about whether the defendant had prepared for, or had practised, inflicting a stab wound at a similar height," he said.

Mr Reeds said Bonser accepted she had killed Casey and said it was likely she would claim the partial defence of diminished responsibility, which would reduce what she did to manslaughter.

But he said the prosecution rejected the claim that the defendant "was suffering from an abnormality of mental functioning which arose from a recognised medical condition".

He said Bonser intended to kill, or seriously harm, Casey and was in control of her actions at the time.

A psychiatrist found "no clear evidence of an acute episode of mental illness", the court heard.

Mr Reeds said: "Deliberately stabbing another person through the abdomen with a large kitchen knife is an action that speaks for itself about the intention of the person who did it."

Bonser, who appeared in the dock flanked by four security guards and wearing a baggy sky-blue T-shirt and grey tracksuit bottoms, denies murder.

Members of Casey's family looked on from the public gallery as the trial began.

Later, Kerry Berry told the court how she came across Casey lying in the park.

Mrs Berry said he first thought it was a pile of bags left by children but then realised it was a girl.

"I sort of got her in a recovery position," she said.

"I moved her hair back and I was stroking her face.

"She was foaming at the mouth and I thought she was fitting."

Mrs Berry said she did not realise Casey had been stabbed and there was not much blood.

In a statement read to the court, Casey's mother, Kerry Day, said her and her daughter had told each other "I love you" as she set off that morning.

Mrs Day said she had warned her daughter about crossing a dangerous road as she left.

The case was adjourned until tomorrow.

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