Police have also taken the unusual step of naming 22-year-old Jamie Marsh-Smith who they want to speak to in connection with the murder. He is known to have links to Tottenham, Hackney, Camden and Kilburn.
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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.
The thief, who used a walking stick, was helped by a man who disabled the lock before pushing the woman away.