The man seen on CCTV is described as dark skinned, with a full beard, wearing a light baggy top with a satchel across his chest.
Detective Chief Inspector Sheila Stewart said: "It is vital we track down this man who was in the precise area at the time Mariana was attacked and killed."
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The father of teenager Stephen Lawrence - who was murdered by a racist gang in London - has been honoured at Portsmouth University.
The girl, whose drinks stand outside her home was shut down, and her father fined £150, have received offers to set up stalls elsewhere.
Connie Yates and Chris Gard said it was "disgraceful" that doctors treating their critically-ill son have been subjected to abuse.