Police searching for missing mother of two Katie Kenyon believe she is dead and have arrested a man on suspicion of murder.
Ms Kenyon, 33, was last seen at around 9.30am on Friday getting into a van with a man in the town of Burnley in Lancashire.
A 50-year-old man from Burnley, who was initially arrested on suspicion of kidnap on Monday, has been further arrested on suspicion of murder.
More than 60 specialist search officers from four police forces are scouring parts of a forest in the nearby Ribble Valley for Ms Kenyon.
Gisburn Forest, popular with walkers and cyclists, is said to be the biggest wooded area in the county at nearly five square miles.
Officers from the Lancashire, Greater Manchester, Merseyside and North Wales police forces are combing the area, assisted by Lancashire Fire and Rescue Service and the Bowland Pennine Mountain Rescue Team.
The suspect is said to be known to Ms Kenyon. Detective are set to ask magistrates for further time to question him.
Lancashire Police said: "Whilst Katie has not yet been found we are now working on the basis that she is sadly no longer alive."
Detective Superintendent Gary Brooks, head of major crime at Lancashire Police, said officers are offering her family support after the latest development.
He said: "While I hold some slim hope that Katie may still be found alive I'm sad to say that Katie has now been missing for over four days.
"Despite extensive police enquiries and media appeals we have not found Katie nor have there been any sightings of Katie."
He added: "We are continuing to search a number of different locations in connection with this investigation, including at Gisburn Forest and I am grateful to colleagues from Merseyside and North Wales, and other emergency service partners who are offering assistance in the these complex and wide-scale searches."
At about 9.30am on Friday, a woman matching Ms Kenyon's description left an address on Todmorden Road in Burnley with a man and travelled about 17 miles in a silver Ford Transit to the Bolton-by-Bowland area of north Lancashire.
She has not been seen since.
Police previously issued a photograph of the van, which was found in Burnley, and are continuing to appeal for anyone who saw the vehicle, registration MT57 FLC, or has dashcam footage to come forward.
Ms Kenyon is described as 5ft 9in and white with shoulder-length strawberry blonde or ginger hair.
She was last seen wearing a blue shirt or jacket and white vest top, black leggings and flip flops.
Anyone with information should call police on 101 quoting log 800 of April 24 or they can contact officers through the major incident public portal.
Alternatively, they can contact independent charity Crimestoppers on 0800 555 111.