A woman was found alone and confused near Oxford United's new Kassam Stadium after a night out with friends.
Police said the 28-year-old was found in a "distressed state", but they do not believe she was sexually assaulted.
They are trying to piece together her movements between her leaving a bar in the city centre and being found at the stadium shortly before 2.30am on Saturday.
Police said the woman had visited various Oxford bars that evening and was seen talking to a group of friends in King Edward Street, at the junction with High Street, shortly after 1.45am.
Her movements in the half-hour between then and being found alone outside the stadium are a mystery to both police and the woman.
Police said they want to speak to the driver of a black cab who was seen by security guards dropping the woman off at the stadium shortly before she was found.
The taxi driver could shed some light on why the woman ended up at the stadium.
Police said the driver is not in any trouble and should contact them as soon as possible.
Anyone with information on the incident is asked to contact Oxford police on 01865 266333, or ring Crimestoppers anonymously on 0800 555 111.
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