A student has been convicted of the murder of his 94-year-old step-grandmother three years after an inquest ruled she was the victim of an accidental house fire.

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.

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.

Darnton’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.

The defendant closed his eyes briefly after the jury foreman returned the unanimous verdict and his stepfather rushed out of the courtroom as other members of the family, who were sitting apart from Mr Gregory, broke down down in tears.

Darnton, a former student at Kendal College and Lancaster & Morecambe College, was arrestedat his family home in Combermere Road, Heysham, in May 2019.

Examination of his mobile phone and laptop found internet searches made after the fire including “I’m a murderer”, “I’m a monster and I’m going to hell”, “I want to cause evil” and “Urge to kill again”.

A month before the fatal blaze – when Darnton was 17 – he also searched “Under 18 murder”.

Giving evidence at his trial, Darnton said the case was “all a big misunderstanding” and his confessions were false.

He said he only wanted to impress his “edgy” friends in the Truth or Dare game and felt he had to say something believable, although he knew it was “messed up”.

Darnton said his comments to the counsellor that he had killed somebody were “attention-seeking” but denied saying he had set his step-grandmother’s house alight.

He claimed his Google searches were related to his interest in killers and true crime, and said he did not even understand at the time the true definition in law of murder.

An investigator from Lancashire Fire and Rescue Service ruled in August 2018 that the probable cause of the fire at Mrs Gregory’s home was a cigarette and told the jury his opinion was unchanged.

However, a fire examination expert for the prosecution said the chances of that were “extremely remote” and he believed the blaze was most likely started by naked flame ignition near the front bedroom window of the property.

The Crown said attempts were also made to hamper any escape by Mrs Gregory, with a table placed in front of the conservatory doors, the unplugging of a phone cable and the dismantling of a smoke alarm.

A packet of Tunnock’s teacakes and a can of Coca-Cola, bought by Darnton from a nearby service station less than two hours before the fire, were discovered at the scene.

Evidence shown to the jury also included drawings found at Darnton’s home address, outlining the floor plan of Mrs Gregory’s bungalow. They contained labels including “good hiding place” and “quick exit”, as well as references to needing a “good alibi”.

Mr Gregory, giving evidence for the defence, said Darnton – whom he home-schooled – had been suffering from depression for a number of years and was “plagued by intrusive thoughts which were very, very disturbing”.

He told the jury he would have taken Darnton to the police himself if he believed he had hurt his mother in any way.

Following the verdict, Detective Chief Inspector Zoe Russo, from Lancashire Police, said: “Darnton regularly visited Mrs Gregory’s home after her son took him under his wing at a young age. Darnton would spend time at Mrs Gregory’s bungalow and not only took advantage of Mrs Gregory’s kind nature, he also used his access to her home to meticulously plan his murderous act.”

Sentencing will take place on Friday.