A 12-year-old boy suffered a head injury and lost his iPhone 4 when he was robbed at knifepoint.
The boy was grabbed around the mouth when he was attacked by four teenagers off Donnington Bridge in Oxford.
He was later taken to the John Radcliffe Hospital and treated for a head injury.
The crime happened in Meadow Lane at 7.45pm on March 19 but police only released the details yesterday.
Officers described the robbers as Asian and aged between 15 and 17. A 17-year-old was arrested on suspicion of robbery on March 19 and bailed until April 22.