Four youths robbed a teenage boy after following him in Corby.
Police are appealing for a potential witness who helped the victim to contact them after the incident on September 8.
The boy was in Oakley Road, close to the Tesco petrol station, between 10.30pm and 10.45pm when he was approached by four unknown youths on bicycles who had been following him.
After asking what he had on him the group robbed the boy of his iPhone, bank card and £15 in cash before leaving the scene on their bikes.
A police spokesman said: "The suspects were all male, in their teens, white and had their faces covered.
"One is described as in his late teens, 6ft, of thin build, wearing a grey top with black sleeves and the hood pulled up, and black trousers. Two of the others wore black tracksuits.
"Three of the group rode BMX style bikes – one had a black frame and green wheels, another had a blue frame, and the third had a black frame.
"The fourth was described as a mountain bike."
After the group left the victim was spoken to by an unknown man who stopped to check if he was okay.
The spokesman added: "Officers would like to speak to this person as a potential witness, and are appealing for witnesses and anyone with information about the robbery to get in touch by calling 101 or Crimestoppers on 0800 555111."