Mary Gregory was discovered under a table in the conservatory of her smoke-logged bungalow in Heysham, Lancashire, in the early hours of May 28 2018 and died four days later in hospital.
An investigation by Lancashire Fire and Rescue Service concluded the most probable cause was a dropped or carelessly discarded cigarette and ruled out any third party involvement, which later led to a coroner recording a verdict of an accidental death.
However, police reopened the case in May 2019 following comments made by Tiernan Darnton, 21, during a counselling session in which he said he killed Mrs Gregory – his stepfather’s mother – by using a lighter to set a curtain on fire on at the address in Levens Drive.
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During the inquiry it emerged that Darnton had made a similar confession several weeks after Mrs Gregory’s death, during a game of Truth or Dare with two friends in which he revealed his “darkest secret”.
He told the pair: “I have a secret I haven’t told anyone. I may have killed someone.”
When pressed, he said he started the blaze because he did not want Mrs Gregory, formerly from Nuneaton, Warwickshire, to suffer any longer from dementia, Preston Crown Court heard.
Darton’s friends did not believe him and he persuaded them to keep quiet, but he told a counsellor in May 2019 about a friend “who could send me to prison cos of what he knows”.
He added that he had felt “powerful” at an event involving a person who had died – thought to be Mrs Gregory’s funeral – as he “knew what had happened and everyone else in the room didn’t”.
The female counsellor ended the session by saying to him “I’m not really clear what you’re saying but I think you’re trying to tell me you’ve killed someone”, to which Darnton mouthed “Yes”.
A week later he told the counsellor and his stepfather, Chris Gregory, 66, that he had started the fire and the matter was later passed to the police by the counsellor, the court was told.
Sentencing will take place on Friday