The 16-year-old boy shot dead in south London on Tuesday has been named as Lathaniel Burrell.
The teenager was killed on Paradise Road, near Stockwell Tube station.
Detective Chief Inspector Sarah Lee said: “This is a horrendous crime, which has shocked the local community and beyond. Our thoughts are with Lathaniel’s family and friends at this devastating time.”
The Metropolitan Police is asking for the public’s help in tracking down those responsible.
Paramedics were being called at 3.21pm, and Lathaniel died while being treated at the scene.
His family are currently being supported by specialist officers.
DCI Lee said detectives are now examining CCTV footage and collecting witness statements – with officers deployed on “reassurance patrol” and engaging with local schools.
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“We need the help of the public too – if you were in the local area please think remember back to yesterday afternoon and come forward with any information, no matter how small,” she added.
No arrests have been made and detectives are keeping an open mind about the possible motive for the attack.
Anyone with information is being urged to call 101.
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A man who said he was Lathaniel’s uncle claimed the gunmen had posed as delivery drivers, and were riding a moped.
Meanwhile, a neighbour described the teenager as “very bright at school”.
A young woman who lives nearby added: “This area is not safe at night. There are a lot of teenage boys that cause a lot of trouble who live around here.
“They hang around at night, play around with the cops. Things like playing with fireworks and running from the police.”
The killing is the first fatal shooting in London so far this year.