A 14-year-old boy has admitted killing his foster carer by stabbing her 10 times in a frenzied attack after he was grounded.

The boy, who cannot be named for legal reasons, was originally charged with murdering Dawn McKenzie, 34, a former nursery worker, but yesterday his plea of guilty to a reduced charge of culpable homicide was accepted by the Crown on the grounds of diminished responsibility.

The High Court in Glasgow heard that in the days leading up to the killing the boy had had his X-box, mobile phone and laptop taken off him. Psychiatrists who examined the boy said he was not able to control his behaviour at the time.

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Mrs McKenzie was stabbed 10 times on the head and body. The fatal blow severed a major blood vessel and caused her to bleed to death. She was also stabbed twice in the scalp and the force of one of these blows was so great the tip of the blade broke off and imbedded itself in her skull.

The defensive injuries she received showed she had fought for her life. After the boy fled, Mrs McKenzie managed to dial 999 and as she lay dying told police who had stabbed her.

Advocate depute Alex Prentice, QC, prosecuting, said: "The victim and her husband Bryan treated the accused as their own. They were all due to go on holiday abroad together. It would appear there was nothing remarkable about his behaviour in the lead-up to this offence."

The court heard it was a normal Friday night, with Mr McKenzie driving Mrs McKenzie to the supermarket, taking the boy along, and then going for a drive before picking her up.

Mr Prentice added: "The accused had a circle of friends and a keen interest in football and is described by his friends as quiet and likeable."

The boy, who was 13, stabbed Mrs McKenzie at Deveron Crescent, Hamilton, in South Lanarkshire, on June 24 last year.

The court heard it was the last day of school and instead of coming home at the agreed time he went swimming and then on to a McDonald's restaurant with friends.

Mr Prentice said: "He did this without permission and was told trust was an important issue. He was grounded and told he had to go to bed an hour earlier as punishment."

The laptop and phone given to him by his mother, and which he used to keep in contact with her, had been confiscated days earlier. Mr McKenzie said the boy did not seem particularly perturbed about this.

Mr McKenzie left the house at about 7pm or 7.15pm and went to his brother-in-law's house to watch a film. He said there was no animosity between Mrs McKenzie and the boy. He kissed his wife and ruffled the boy's hair as he left and said: "See you later wee man."

That was the last time he saw his wife alive. She and the boy were both sitting watching television.

At 8.06pm a 999 call was received from Mrs McKenzie, who said she had been stabbed. In the 999 tape she can be heard shouting "Mum" and "help, help," and "Mam, am I dying?"

During the call, Mrs McKenzie said she couldn't breathe.

Mrs McKenzie also managed to dial her mother Ray Byrne's number and left a message.

Mr Prentice said: "On the voicemail recovered she is heard to say: 'Mum' and 'help'. When police arrived they were let into the block of flats by neighbour Catriona McLachlan, who went to help Mrs McKenzie as they waited for paramedics.

The police found the front door ajar and Mrs McKenzie lying covered in blood on the living room floor.

A blood-stained knife with the tip broken off was lying on the couch. One of the police officers asked Mrs McKenzie: "Who stabbed you," and she said it was the boy.

She was asked where he was and replied she didn't know. He was stopped by a police officer at 8.25pm in Wellhall Road, Hamilton, who saw him walking along with a cloth wrapped round his hand. He asked him "What's wrong with you wee man, you're shaking?" and he replied: "I've just stabbed my foster carer."

He said he had done it with a knife and told the police officer it was still in the house at Deveron Crescent. When interviewed by police, the boy admitted stabbing Mrs McKenzie on one occasion. When he was charged with murder, he said: "I only stabbed her once."

Mrs McKenzie died in Hairmyres Hospital, East Kilbride, at 9.40pm. A post-mortem examination revealed the wound to her stomach damaged the small bowel and cut through one of the main blood vessels and she died from loss of blood.

Lord Pentland was given victim impact statements from Mrs McKenzie's husband and mother and said: "I shall read these documents carefully before I come to impose sentence in this case." Sentence was deferred on the boy, represented by Donald Findlay, QC, until August for background reports.