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The Canterbury teenager went missing from her Ashburton home on New Years' Eve in 1998.
Police are now offering a $100,000 reward for anyone with critical information.
Detective Inspector Greg Murton said the focus of the investigation has shifted away from her family.
"My personal view is that it was probably a stranger ... someone who most likely is very familiar with the area ... she lived in the in the Riverbank area, rather than a person passing through that. They're also very familiar with the Rakaia Gorge area and probably a very good chance that they were involved with cannabis growing or cannabis usage."
Bentley failed to return home from walking her dog and 18 days later her body was found near Rakaia Gorge, close to a cannabis growing area.
Police do not believe that was where she was killed.
"The most likely scenario in my view, is that she was taken while she was on the dog walking route and and held somewhere and then her body was taken up to the gorge afterwards."
Journalist Chris Cook has been following the cold case closely for the past decade. He published new information on the case in this week's Listener.
Cook told Morning Report there was no evidence to support that the Bentley's father and brother - who were previous suspects - were involved.
"No evidence, there was no blood at the house. No blood in the father's ute ... he was accused of transporting his daughter's body to the gorge."
He said police believed it was "more likely that it was a stranger abduction, more likely a sexual assault and murder committed by someone who was a cannabis user or a local familiar to the area".
Cook said police would not divulge details, but the areas where the dog and underwear were found are places frequented by cannabis users.
Another new finding was that the father was spotted in Lyttelton on the afternoon of the incident, and he could not have gotten back in time to be involved.