A 14-year-old boy was found hanging in his bedroom by his younger sister.
Laura Plowman found her brother Grant at 7.30pm on Saturday while the rest of the family were downstairs at their home in Woburn Road, Gossops Green, Crawley.
Paramedics were unable to resuscitate Grant and he was pronounced dead at Crawley Hospital's accident and emergency department at 8.25pm.
Grant's stepfather Bob King said: "It's devastating for us. I have no idea how it happened. We really don't know."
Yesterday the curtains were drawn at the house and neighbours said they were deeply shocked.
Ron Bessell, 62, said of Grant: "He just seemed quite a happy-go-lucky sort of a boy. He was a popular lad who would play football with his group of friends. He was always kicking a ball around outside."
Mr Bessell's wife Sandra, 59, said Mr King came round on Saturday night to tell them what happened.
She said: "He just came in and explained what had happened and asked us to keep an eye on the house.
"It's just so distressing. I think it was an accident, I don't think it was intended at all.
"It was probably just a prank that went wrong. The kids were just normal like any other kids."
Detective Sergeant Michael Beard of Crawley CID said: "No other persons are being sought in connection with this incident."
A post-mortem examination is due to be carried out tomorrow.
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