A teenager has been arrested after a 12-year-old boy said he was punched in the face and had his bike stolen in St Albans.
Police are looking for a dog walker as they believe she may have witnessed incident on the Alban Way cycle path.
The victim is believed to have had his bike taken by another teenager who he had been with on Saturday evening at around 6.50pm.
The victim said he chased after the boy and was subsequently punched in the face before the offender rode off on the bike. The victim was not injured.
PC Francis Mungai is investigating and said: "I would like to appeal to the lady who was walking a dog, which is possibly a boxer dog, in the area at the time and walked past as the incident took place, to contact me on 01923 271613 or via the Herts Police non-emergency number 101 quoting crime reference number F1/14/2539."
A 13-year-old boy from Hemel Hempstead has been arrested in connection with the incident and is currently on police bail pending further inquiries. He will return to Watford police station on August 20.