Detectives are appealing for information following a report of a serious sexual assault in the Strand Road area of Derry in the early hours of Thursday 15 December.
It is believed that sometime between 1.30am and 1.45am, a young woman was assaulted by an unknown male in an alley in the area.
He is described as being approximately 6ft 3" to 6ft 4" tall, of stocky build, with long brown hair and spoke with a Donegal accent.
Anyone with information about this incident is asked to contact Detectives in the PSNI's Rape Crime Unit in Ballymena on 0845 600 8000. Or if someone would prefer to provide information without giving their details, they can contact the independent charity Crimestoppers and speak to them anonymously on 0800 555 111.