A teenager was robbed for his mobile phone by a thug who claimed to have a knife on a London Underground train.
British Transport Police said the male suspect approached his young victim after boarding the service at South Harrow station on the Piccadilly line.
A force spokesman said: “The man intimated that he had a knife on him, revealing a wooden handle protruding from his waist band, and demanded he hand over his phone.”
The incident happened at around 4pm on Saturday, February 7.
Detectives released CCTV images of a man - wearing a black bomber jacket and baseball cap - who may have information that could help their investigation.
Anyone who recognises him is asked to contact British Transport Police by texting 61016 or calling 0800 40 50 40, quoting reference 476 of 7 February.
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Alternatively, they can contact Crimestoppers anonymously on 0800 555 111.