The incident was reported to have happened near the Guildhall Square Cenotaph on New Year’s Day. On Friday police released a picture of a man to whom they wanted to speak.
Today they say they have identified him. Nobody has been arrested.
Between 3am and 4.20am, an 18-year-old woman was sexually assaulted by a male stranger.
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The woman is being supported by specialist officers.
Officers are still appealing for information.
A statement from Hampshire Constabulary said: ‘We have been conducting a number of lines of enquiry as part of our investigation into this incident.
‘We believe a group of people assisted the victim after the incident, was this you?
‘We believe there may have been a number of people in the area at the time following New Year’s Eve celebrations and would like to hear from anyone with information that may assist our enquiries into this investigation, no matter how small it may seem.
‘We understand that incidents of this nature can be concerning, however we’d like to reassure our residents that a thorough investigation is underway and we believe this to be an isolated incident at this time.’
Anyone with information about the incident should call 101, quoting the reference number 44220000434.
People can also go online and submit information via a form here.