A pair of teenage boys killed two 12-year-old girls and an adult man by trapping them inside a cave along with a burning sofa after a dispute over four euros (£3.50, $4.70), a court heard.
The boys, 16 and 17 respectively, were said to be angry after the man, 21, did not pay back the small change that one of them had lent him.
The man entered the cave, which contained an abandoned sofa and was a popular hangout for locals in Andalusia, Spain.
He did not come out alive.
The 16-year-old set fire to the furniture and then barricaded the entrance to the nook with an old door and industrial strength tape.
The defence claims that the boys had no idea two 12-year-old girls were already inside the cave, which fast became a furnace.
Despite the tormented screams of the three people trapped inside, the boys did not let them out, Spanish news site The Olive Press reported.
Lawyers said the 17-year-old tried to free the victims when he realised how hot it had become behind the door but the 16-year-old, who remained "calm and quiet", threw stones at him to prevent the relief effort.
A Spanish prosecutor is seeking eight years in a young offenders' institute and five years probation for the 16-year-old and five years in the institute plus five probation for the older suspect.
Three other boys face lesser charges of being present when the killings took place in April and doing nothing to prevent them. They could each be incarcerated for two years.
A judge refused to reduce the sentences sought by the prosecution, despite the defence's claim that the boys were "not monsters" and that the entire incident was a prank that "got out of hand".
The trial continues.