The search for missing mother-of-two Nicola Bulley will have "quickly escalated into a high-risk missing persons investigation", a former senior detective has told Sky News.
Howard Millington, who worked on many missing persons cases during a 30-year career with Greater Manchester Police, said Ms Bulley "seems to have disappeared into thin air".
The former detective superintendent described the case as "very unusual" and "very concerning", adding: "It will have been quickly escalated, I've no doubt.
"Lancashire Police have spared no expense in resourcing this properly."
There will be different strands to the investigation, which will be "structured, detailed, and scientific", Mr Millington said.
There will be also be several "working hypotheses", while a police search adviser will have been called in.
Ms Bulley, a 45-year-old mortgage adviser from Inskip, was last seen on Friday 27 January at about 9.15am on a footpath by the River Wyre in the village of St Michael's on Wyre.
Her mobile phone, which was still connected to a conference call when it was discovered, will be a "key part of the investigative strategy", Mr Millington said.
Forensics officers will have dusted it for fingerprints and trace evidence.
Ms Bulley's call data will also be examined. Anyone in touch with her recently will be contacted to see if they can "offer any information".
Her friends and associates will be interviewed to get an impression of her state of mind.
A key part of the investigation will involve the river. Mr Millington described it as "quite a fast-flowing, wide" stretch of water with steep embankments nearby.
One of the working hypotheses will be whether Ms Bulley went into the river. Its flow will be studied.
Detectives will also be considering whether any "foul play" was involved, in which case, are there any witnesses?
CCTV will be examined and officers will go from door to door, Mr Millington said.
Hospitals will have been contacted in case Ms Bulley was ill and taken for treatment by a stranger.
Her heartbroken sister and father say her "children need her home" and have described being "stuck in a nightmare" since her disappearance.
A potential witness has now been identified, officers have said.