Police in England investigating the disappearance of mother-of-two Nicola Bulley say their "main working hypothesis" is that she fell into the River Wyre while out walking her dog.
Ms Bulley, 45, might have been trying to deal with an "issue" with her springer spaniel Willow, causing her to go to the edge of the River Wyre and into the water, the officer in charge of the missing person investigation said.
Ms Bulley's family and friends had made desperate pleas for information during the week-long hunt after she vanished.
Minutes before, she had dropped her daughters - aged six and nine - at the local school in St Michael's on Wyre, Lancashire, last Friday morning.
The mortgage adviser, from nearby Inksip, then went for her normal walk along the River Wyre, letting Willow loose.
But in a ten-minute window between her last being seen and from police tracing telephony records of her mobile phone not leaving the bench it was placed on overlooking the river, at 9.20am, officers believe she may have fallen into the river.
Ms Riley said: "Our main working hypothesis is that Nicola has suddenly fallen into the river, that there's no third party or criminal involvement, and that this is not suspicious but a tragic case of a missing person.
"This is particularly important because speculation otherwise can be really distressing for the family and for Nicola's children."
She said an "issue" with Ms Bulley's dog may have caused her to place her phone on the bench and go to assist Willow.
Ms Riley continued: "I don't wish to speculate, in that we don't know, but it is possible as the dog was loose and off the lead.
"There may have been an issue with the dog that led her to the water's edge, she puts her phone down to go and deal with the dog momentarily, and Nicola may have fallen in.
"We assume the dog didn't get into the river, but we don't know why Nicola may have if she did."
The officer said a massive amount of police resources had been used to search the surface and under the water of the river and surrounding farmland - including divers, drones and helicopters.
And after tracing local CCTV, dashcam and doorbell camera footage, police are confident Ms Bulley did not leave the area near the river.
Ms Bulley's neighbours, friends and local community had carried out searches of the area and appealed for information on possible sightings that would bring a "glimmer of hope".
The disappearance caused huge public interest and desperate pleas for information by the family. Ms Bulley's partner, Paul Ansell, said he would "never lose hope" of finding her.
Police vowed to continue the search for Ms Bulley, to "bring her home".
They have urged the public to look out along the river for the items of clothing that Ms Bulley was last seen wearing, including an ankle-length black quilted gilet jacket, a black Engelbert Strauss waist-length coat, tight-fitting black jeans, long green walking socks, ankle-length green Next wellies, a necklace and a pale blue Fitbit.