The shot was heard by 12-year-old Elizabeth Cox at about 3.30pm as she arrived home with a friend. She rushed inside to find her brother, Billy, dying on his bedroom floor from a bullet wound to the chest. The 15-year-old is the third teenager in 11 days to be shot dead in south London.
Fenwick Place, a collection of grey Sixties flats and maisonettes where the Cox family had lived since Billy was eight, stands just yards from Clapham High Street - a strip of trendy bars and restaurants and desirable Victorian terraced homes.
But on Wednesday afternoon, it was a 12-year-old girl running with bloodied hands into the courtyard outside her home that told how grim life can be on the nearby council estates - a place where schoolboys are now shot dead in their homes.
Jeanine Easton, a neighbour, said: "Elizabeth came screaming out of the house with blood all over her hands. We asked her where she was cut because of all the blood on her. All she could say was that it was her brother and he had been hurt."
Efforts by an ambulance crew to revive Billy failed and he was declared dead at the scene. It is understood his mother, Kim, who is Thai, only found out about the killing when she arrived home an hour later.
Detectives were yesterday trying to piece together the sequence of events that led to Billy being shot dead on the ground floor of his family's three-bedroom maisonette.
Scotland Yard said that Elizabeth, a keen ballet dancer, had almost certainly disturbed her brother's killer or killers. It is understood that they escaped through the open back door and there was no sign of forced entry, suggesting that Billy knew his killers.
The officer leading the murder hunt said they were still looking for a motive. Detective Chief Superintendent Helen Ball, head of the Operation Trident investigation into gun crime, said: "[Billy] was in his home with the suspect or suspects. We are trying to find out if anybody had a motive for trying to kill Billy. This 15-year-old boy was killed in his own home. We need everybody's help to solve this murder."
Police sources said a direct link with the two other murders of teenagers in south London had not been ruled out, but was unlikely.
The Independent was told by neighbours of a fight at the Coxes' home this week, in which Billy had come to blows with an unnamed youth. Joanna Harrison, whose 15-year-old daughter was a close friend of Billy, said: "Billy had words with one lad and they swapped a couple of punches."
Neighbours and friends spoke fondly of a rumbustious and popular boy who had grown up on the estate, regularly attending the local adventure playground and outh centre where he indulged his great sporting passion - table tennis.
His English father, Tom, a builder, and his mother, a waitress, moved to the estate in 1997 and they were considered a quiet, hard-working family.
But more recently, Billy had changed. Last year he was enrolled at Fairbridge, a specialist education centre for "challenging" young people, although he was not formally excluded from his school, Ernest Bevin College in neighbouring Wandsworth.
Having expressed an interest in going into the construction trade, he was enrolled on an eight-day course at Fairbridge last month but attended only two days.
Instead, there is evidence that Billy had fallen into a cycle of minor offending. He was under a supervision order and had been electronically tagged.
His graffiti "trademark", Remer, is to be found in numerous places on the estate, including a garage door just yards from his home.
A card with one of the bouquets of flowers laid by distraught teenagers from the Lambeth Academy yesterday read: "To Remer, nuff wiv. All da man dem will miss u, heaven's new fallen soldier."Reuse content