Officers are today releasing CCTV images following a robbery at West Dulwich Railway Station.
The incident happened at around 4pm on Tuesday 27 June.
Two men approached a teenage boy and asked him to follow them on Instagram. When the boy took his phone out, they snatched it from him. When the victim tried to take the phone back, one of the men verbally threatened him by saying that he had a knife.
On the same day, the men used the same tactic to carry out another robbery and attempt one more.
Detectives would like to speak to the men in the image, who they believe may have information that could help their investigation.
Anyone who recognises them is asked to contact British Transport Police by texting 61016 or by calling 0800 40 50 40 quoting reference number 455 of 27 June.
Alternatively, you can call Crimestoppers anonymously on 0800 555 111.