A 21-year-old student who went missing in the early hours of Boxing Day was suffering from depression and it is "most likely" that she has gone into the sea, police have said.
Sophie Smith did not have her phone with her and has not accessed her bank or social media accounts since leaving home.
She was believed to be wearing only a vest and shorts when she left her home in Gorleston, near Great Yarmouth, Norfolk, at around 3am on December 26.
Norfolk Police said further sea and coastline searches would take place on Friday.
The University of East Anglia student had been suffering from severe anxiety and depression, and police believe the "most likely scenario" is she entered the sea at Gorleston.
Officers have sought expert analysis of tidal movement and will conduct sea and coastline searches accordingly.
Inspector Will Drummond said: "Sophie had been receiving treatment for depression and anxiety.
"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.
"The sea searches will be carried out by the coastguard while the police helicopter is being deployed to conduct coastline searches.
"Based on tidal analysis, the searches will be carried out in targeted areas."
Sophie went missing from her family home in Avondale Road at around 3am and was captured on CCTV heading towards the seafront a short time later.
Norfolk Police said there were no other confirmed sightings.
"Local CCTV footage has been examined but unfortunately Sophie is not captured on any other cameras," said Mr Drummond.
"She didn't take her mobile phone and hasn't accessed her bank accounts or social media accounts since leaving home.
"This is clearly a very distressing time for her family and we continue to keep them updated of ongoing work in this case."
Police earlier said that "the hope of finding Sophie safe and well diminishes as time goes on".
A Facebook page called Find Sophie Smith has gained more than 23,000 followers, and volunteers have helped co-ordinate their own searches.
Sophie is described as white, about 5ft 8in and slim with shoulder-length blonde hair.
Anyone with information is asked to call Norfolk Police on 101.