A student who went missing from her home on Boxing Day was suffering depression and it is "most likely" she went into the sea, police have said.
Sophie Smith, 21, was wearing a vest and shorts when she set out in very cold weather at about 3am on 26 December.
CCTV images showed her heading towards the seafront near her family home in Gorleston near Great Yarmouth.
Police have been searching the Norfolk coast and sea in the hopes of finding the University of East Anglia student.
They say that Sophie, who has been experiencing severe anxiety and depression, did not have her phone with her and has not accessed her bank or social media accounts since she vanished.
Inspector Will Drummond said: "Our inquiries have revealed there's been a degree of pre-planning to her disappearance and this, combined with her mental state and comments made the previous day, suggest the most likely scenario is that she has gone into the sea."
He went on to say that expert analysis of the tide is being carried out.
Sophie has not been captured on any other CCTV cameras.
Inspector Drummond said: "This is clearly a very distressing time for her family and we continue to keep them updated of ongoing work in this case."
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A Facebook page called Find Sophie Smith now has more than 23,000 followers and volunteers are co-ordinating their own searches.
Anyone who may have information is asked to call Norfolk Police on 101.