Missing Nicola Bulley could have been abducted and held somewhere, Peter Faulding admits, as he said her family have been told to "prepare for the worst".
The private investigator said it was "crazy" that Lancashire Police has not publicly acknowledged other reasons for her disappearance after the 45-year-old vanished more than two weeks ago.
Mr Faulding, chief executive of Specialist Group International, who was taken on by the family to search for the mum-of-two, said all possibilities needed to be looked at after searches of the River Wyre have drawn a blank.
The investigator has already told reporters he does not believe Nicola is in the water, saying she would have been "shoved really hard" if so.
"The police divers who are highly professional, searched the river thoroughly that day, thoroughly all the way down that strip, and from all my experience of looking at cases like this, we find people where they go down," he said.
“And that's a fact. They don't drift unless it's a really, really heavily flooded river when they move down.
"But it was not at the time and that was confirmed by the family. I don't think there's any evidence to rule anything out," he told GB News.
"I mean, I've already been very blunt with [Nicola’s partner] Paul, and I've spent a lot of time with Paul and just said to them, we've got to potentially be prepared for the worst here."
His comments came as it emerged on Tuesday that police were probing who had angling licences to fish in the river when Nicola disappeared on the morning of Friday, January 27.
The mortgage advisor was last seen walking her dog Willow by fellow walkers in the village of St Michael's on Wyre, with her phone found on a bench by the river a short time later.
Mr Faulding, who has also spoken to Nicola's family about searching nearby deposition sites, said "we've just got to say it as it is".
"And I've said to him, we could be looking for a body. We could be looking for a deposition site here.
"She could have been kidnapped. She could have been held against her will and this is what needs to be really well looked at.
"There's a lot of work going on behind the scenes."
He added: "I’m not here to beat the police. I work with them all the time.
"They do a great job and they thought right and good, but I think on this particular inquiry, the media aren’t being told, the public's not being told, they need to get a grip of their PR."
Mr Faulding said: "I was with Paul and he's pleading with them to actually search in the surrounding buildings and they confirmed that had not been done.
"There's been a lot of people turning up trying to break into buildings, please don't do that, please stay away and let the police do their job.
"Obviously that does need to be done thoroughly now because we don't know where Nicola is.
"I'm praying that Nicola has just walked off somewhere and will show up and she's okay, she just walked away but there's no no sign of that, she was a very happy girl, she was a creature of habit."
Asked about Nicola’s partner Paul, he said: "He actually is going mad to be honest, he's holding it back.
"I was on the phone again to him today…he just wants more land searches done.
"I mean, I've worked on hundreds of missing people cases in 20-odd years. The surrounding area should have been beaten down.
"So it's every nook and cranny, every drain, not just focus on the river."
He added: "It's crazy that we're just saying she's in the river. I may be wrong, she may end up at sea.
"But for all my experience, this will be the first where I'm actually wrong. Normally I'm right on this type of thing with my experience."
