A man was knocked unconscious and had cash stolen at an event at The Racecourse in Northampton last month.
The man was close to the band area in the centre of the park when he was attacked by two or three men, who kicked and punched him, causing him to lose consciousness. They then stole cash.
The incident happened at the Umbrella Festival on August 19 sometime between 10pm and 11.45pm (police released information today, September 11).
Witnesses, or anyone with information, are asked to contact Northamptonshire Police on 101. Alternatively, they can call independent charity Crimestoppers anonymously on 0800 555111.