Four siblings died after a 'drunk and stoned' teenager deliberately started a fire in a bedroom wardrobe, a court has heard.
Dyson Allen, 19, is charged with the murder of Reece Smith, 19, four-year-old twins Holly and Ella and Jordan, two, in a blaze at their Freckleton home on 7 January last year.
He was the only other person upstairs at the dormer bungalow when the fire broke out, Preston Crown Court heard.
He denies four counts of murder. He also denies four alternative counts of manslaughter.
Neil Flewitt QC, prosecuting, said: "It is the prosecution's case that the defendant, Dyson Allen, was responsible for starting that fire and he did so deliberately."
The fire began inside a wardrobe in the bedroom shared by the three young children and their mother, Michelle Smith, said the prosecutor.
Three family friends were also in the house, along with the defendant, who was a friend of Ms Smith's 19-year-old son Reece.
Allen was a regular visitor and stayed overnight the evening before the blaze, the jury was told.
Mr Flewitt said Ms Smith had allowed Allen to store some of his personal belongings in a bag in the young children's bedroom.
"The presence of that bag may be significant because it would provide a reason for Dyson Allen to go into that bedroom," he said.
"Apart from the three young children who were asleep in their beds, he was the only person upstairs when the fire started and he was the person that alerted the other occupants of the house to what was happening."
Mother-of-nine Ms Smith, 37, said the lights went off before the defendant jumped down the stairs and shouted 'fire' before running through the kitchen and out of the back door.
According to a witness, the defendant appeared 'drunk and stoned' at the time of the fire.
And in a witness statement given to police in the early hours of January 8, Allen said he began celebrating Ms Smith's birthday at lunchtime by drinking from a bottle of vodka.
The trial continues.