Mayor of London Boris Johnson said:
"It is heart breaking to see the life of another young Londoner taken so tragically. My thoughts are with this young man's family at such a terrible moment."
"This horrific incident serves as an unwelcome reminder that although violent crime in the capital continues to fall, we must continue our fight to tackle youth violence and get knives off our streets."
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Rebecca Casserly was on her way to buy a coffee in north London when her phone was suddenly snatch from her hand by a man riding a bike.
Lee Curley took more than 120 pictures and videos of his attacks on the girl, said to be under the age of 13.
Council workers uncovered the AK-74 concealed in the undergrowth.