Police investigating the Katie Kenyon who was reported missing from home have arrested a man on suspicion of kidnapping and further arrested on suspicion of murder.
Katie Kenyon, 33, was last seen on Friday getting into a Ford Transit van in Burnley, Lancashire.
While she has not been found, police said they were now working on the basis that she is no longer alive.
A 50-year-old man from Burnley is being questioned and remains in custody, Lancashire Police said.
The force said it was making an application to Burnley Magistrates’ Court for permission to hold him until 21:00 BST on Wednesday.
Ms Kenyon’s family have been informed of the latest development and are being supported by officers, police added.
Det Supt Gary Brooks said: “While I hold some slim hope Katie may still be found alive I’m sad to say she 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 her.”
He said Ms Kenyon’s family were “understandably extremely upset”.
“We are continuing to search a number of different locations in connection with this investigation.”
Members of the public were asked not to attempt to join the searches as the area was “very large and remote with no phone signal”.
Det Supt Brooks added: “I would hate for someone to get lost or come to harm while trying to help. Please leave it to us.”
Police said Ms Kenyon was last seen leaving an address in Burnley before getting into a van registered MT57 FLC at about 09:30 BST and travelling to the Bolton by Bowland area of north Lancashire.
An image of the silver van has been released as officers appealed for information.
Ms Kenyon was described as 5ft 9in with shoulder length strawberry blond highlighted hair and was last seen wearing a blue shirt and white vest top, black leggings and flip flops.