Investigators may be getting closer to finding the killer of Ashburton schoolgirl Kirsty Bentley

Police are refocusing on the case -- offering a 100-thousand dollar reward to finally solve one of New Zealand's highest-profile murder mysteries.

They believe the killer is one of their 30 to 40 prime persons of interest.

Hundreds of local men have been spoken to by police since the 15-year-old disappeared while walking her dog on New Year's Eve in 1998.

Now, Detective Inspector Greg Murton has developed a "stranger-type abduction" theory - involving a lone killer who lived locally - and either smoked or grew cannabis.