A 16-year-old girl was sexually assaulted by a man in the crowd at Larmer Tree Gardens.
The incident took place shortly after 9.30pm during the End of the Road Festival Saturday night main stage headliner on September 3.
The teenage victim was assaulted in the front row during the first track of the band's set.
She had been waiting at the barrier since around 8.30pm.
Police have described the man as white, over 6ft tall with short grey hair aged between 40-60.
He was wearing a black t-shirt with bold text and blue jeans.
The victim sought the help of security straight after the incident but the suspect was not located.
Wiltshire Police said the event organisers are fully engaged with its enquiries and are assisting investigating officers.
Anyone with information should contact police on 101 and quote crime reference number 54220093291.
You can also contact CrimeStoppers anonymously on 0800 555 111
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