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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A parent who drove at a teacher because he wouldn't allow access to the school car park, carrying him on his bonnet, has been jailed.
Police shut down roads and evacuated homes near the suburban embassy as they attended to the report of a "suspicious item".
He told her "If you don't be quiet I will kill you" before stealing a bangle and rings from her hand.