Jessica McCagh died of her injuries at Ninewells Hospital
A scientist has told a murder trial that a fire which led to the death of teenager Jessica McCagh was caused by a naked flame.
Forensic expert Rebecca Peplar said the suggestion that debris from a cannabis joint had ignited the blaze was "so unlikely that it can be dismissed".
Miss McCagh, 17, received 85% burns in a blaze at the home she shared with boyfriend Stewart Blackburn, 18.
He denies murder by dousing her in petrol and lighting it.
Ms Peplar also told the High Court in Livingston that tests of the couple's Arbroath flat indicated that the door to the bedroom, where the fire took place, could have been forcibly shut during the blaze.
She said that the handle on the door had signs of mechanical damage, which she said was due to force.
The witness said it was "entirely consistent" with the suggestion that the bedroom door had been held shut.
Describing the blaze as "rapidly developing", she said: "The amount of heat produced would have been considerable."
Her evidence came as jurors watched a film of a series of experiments where scientists attempted to ignite petrol vapour using cigarettes, joints and a lighter.
The trial heard previously how Mr Blackburn confessed in a police interview to "slapping" a jerry can of petrol at Miss McCagh during a row.
He said that after the argument he lit a joint which ignited the blaze on 25 April.
Jurors watched footage which showed that cigarettes and cannabis joints did not ignite the petrol vapour.
The only thing which set the petrol on fire was a naked flame from a cigarette lighter held about a centimetre away, the court was told.
"It is my opinion that the fire was caused by a naked flame," Ms Peplar told the court.
Asked about the amount of petrol she believed was on Miss McCagh and at the scene of the fire, she replied: "I would say at least a litre. I would probably say it's more than that but I wouldn't like to quantify."
The trial before Lord Bracadale continues.