A 16-year-old girl who smothered a seven-year-girl and slashed at her body with a Stanley knife was suffering from delusional beliefs about people around her not being human, a judge has been told.
The teenager who killed Katie Rough in York in January may have been trying to prove the youngster was not a robot, as she had "irrational beliefs" about how people "may not be human and may be controlled by a higher and hostile force", a court heard.
Katie was found with severe lacerations to her neck and chest on a playing field and did not respond to frantic attempt to revive her.
But a judge at Leeds Crown Court heard how she actually died from being smothered by the girl, who cannot be named for legal reasons.
The teenager was standing in a nearby cul-de-sac, covered in blood and carrying blood-stained Stanley knife as she rang 999 to tell police what she had done.
The defendant appeared by video link on Monday where she denied murder but pleaded guilty to manslaughter by diminished responsibility.
This plea was accepted by the prosecution.
The judge, Mr Justice Soole, was told how the girl began suffering from mental health problems more than a year before the killing.
Graham Reeds QC, prosecuting, said she had reported delusional thoughts as well as depression, self-harm and suicidal thoughts.
Watched by Katie's parents, Alison and Paul, Mr Reeds told the court how the defendant had developed severe problems during 2016.
The prosecutor said the girl had talked of being convinced that people "weren't human and were robots".
He said the girl got distressed when one doctor asked her later "whether she killed Katie to test whether she was a robot".
Mr Reeds said, although psychosis was being investigating prior to the killing, it had not been diagnosed.
Nicholas Johnson QC, defending, said it may be his client was "driven by the irrational belief (Katie) may not have been human and needed proof of this".
He said the teenager had thoughts that people around her "may not be human and may be controlled by a higher and hostile force".
The barrister said his client had posted a picture on social media two days before the killing with a concerning message.
He said: "She was clearly crying out for help and support."
The dark-haired girl appeared on the video link sitting next to a solicitor and wearing a black hoodie.
Her solicitor, sitting next to her, confirmed her name when asked by the judge.
Mr Johnson asked the court if the charge of murder could be put to the girl again and she wrote her plea on a piece of paper.
Her solicitor told the court: "I can confirm she has indicated not guilty to murder but guilty to manslaughter."
The judge said he wanted more questions answering by the medical experts before he could pass sentence.
He apologised to Katie's family for the delay and said the case will be adjourned until July 20.