The search for missing mother-of-two Nicola Bulley in Lancashire has entered its thirteenth day, and questions are mounting.

Police think the mortgage adviser tragically fell into the water while walking her dog along the River Wyre.

But family and friends have questioned the theory, and diving experts have been recruited to help get to the bottom of the mystery.

Nicola, 45, of Inskip, Lancashire, went missing on January 27 in St Michael’s on Wyre after dropping her daughters, aged six and nine, off at school.

Lancashire Police say she took her spaniel, Willow, for a walk along the path by the River Wyre, heading towards a gate and bench in the lower field.

Nicola sent an email to her boss, followed by a text message to her friends to book a playdate at 8.57am, before logging on to a Microsoft Teams call.

Her phone was back in the area of the bench before the Teams call ended ten minutes later, at 9.30am, with her mobile remaining logged on after the call.

Another dog walker found her phone on a bench beside the river, with Willow darting between the two.

According to police, she was wearing a black Engelbert Strauss coat, black jeans and had long green walking socks tucked into her trousers under ankle length green wellington boots.

Her hair was tied into a ponytail and she was wearing a pale blue fitbit.

Lancashire Police has said it was working on the hypothesis that Nicola may have fallen into the River Wyre.

Superintendent Sally Riley urged against speculation, but said it was “possible” that an “issue” with Nicola’s dog may have led her to the water’s edge.

The force has been working with the Coastguard, Lancashire Fire and Rescue and underwater experts to search the river and riverbank using sonar, pole cameras and underwater drones.

Police say they will also be searching an area upstream, where the River Wyre empties into the sea at Morecambe Bay.

Police have described the search for Nicola as “unprecedented”, with a team of 40 detectives investigating 500 different lines of inquiry and police receiving “thousands” of pieces of information.

Family and friends have questioned the police theory that she fell into a river.

In a Facebook post, her sister Louise Cunningham urged people to “keep an open mind” as there is “no evidence whatsoever” to support the hypothesis.

A family friend, Heather Gibbons, has said “nothing is making sense” and that speculation, rife on social media, about the disappearance, was “hard” for the family to take.

She said her disappearance has led to members of the public arriving from far and wide, some bringing children and taking selfies, making the area feel like a “tourist spot”.

Nicola’s two daughters “desperately” miss their mother and “need her back”, her partner, Paul Ansell, said in statement last week.

Peter Faulding, leader of underwater search experts Specialist Group International (SGI), has been searching the river for two days after being called in by the family to help.

Faulding, who is sometimes used by police, has said if his team cannot locate Nicola in the river then she is not there and he would not rule out “third party involvement” in her disappearance.

He has suggested the phone being left on a bench overlooking the river could be a “decoy”.

Faulding said that in his 20 years of search experience he had never seen such an “unusual” case and was “baffled” as to what had happened.

He said: “If Nicola is not in that stretch of the river my view is there could be a third party involvement and this (phone) was a decoy placed by the river.”

Questions have also being raised about gaps in CCTV coverage of the area where she vanished from.

A camera close to where Nicola vanished was not working at the time of her disappearance, and the force says it is still a “possibility” she left the area by one path not covered by cameras which is crossed by the main road through the village.

Officers are trying to trace dashcam footage from 700 drivers who passed along the road at the time she disappeared.

Police acknowledge there is a ten-minute window where they cannot account for Nicola’s movements: between 9.10am, the last confirmed sighting, and 9.20am when Nicola’s phone is believed to have been on the bench.

But police have rejected suggestions that Nicola could have been a victim of crime.

Superintendent Riley said “every single” potential suspicion or criminal suggestion that had come in had been looked at by detectives and discounted.

She said: “I would like to reassure the community that nothing in this investigation so far, it has been checked out if it has come in suggesting crime, it has been checked and discounted.

“So every single potential third party line of inquiry and potential suspicious or criminal element has been looked at and discounted.

“It does remain our belief that Nicola sadly fell into the river and that this is a missing persons inquiry.”

The National Crime Agency had also looked at the investigation by Lancashire Police and had also failed to identify any other suspicious line of inquiry, she said.

Riley told reporters at a press conference in the village that Faulding is not included in “all the investigation detail”.

She said: “Our search has not found Nicola in the river and then a re-search in parts by SGI has found the same.

“That does not mean … that Nicola has not been in the river.”