Neighbours of eight year old Imani Green from Balham have spoken of their sorrow following the girl's death.
She was shot dead in the town of Duncans in Jamaica on Friday night.
One friend of the family said: "This is a close-knit community, so this is going to hurt us."
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Thousands of passengers now face delays, with Southeastern trains warning "all services" would be affected.
Paramedics attended a scene in Willesden, north-west London, on Monday afternoon where they found the teenager with stab wounds.
At least five people have suffered minor injuries but the cause of the blast is still unknown.