Forty sites are to be searched by specialist police officers in a new bid to find the remains of a Co Tyrone teenager who went missing in 1994.
The major operation to locate the body of 15-year-old Arlene Arkinson could last a period of months.
The teenager, from Castlederg, went missing 17 years ago after a night out in Bundoran, Co Donegal.
Police believe she was abducted and killed.
She was last seen in a car driven by convicted child killer Robert Howard. In 2005, he stood trial for her murder despite the fact her remains had not been found.
The 77-year-old was acquitted by a jury which was not made aware he was serving a life sentence for the 2001 murder of 14-year-old Hannah Williams from south London.
The PSNI has spent the last six months planning the fresh searches in and around the Castlederg area.
As well as specialist forensic officers and sniffer dogs, the operation is being assisted by geologists, archaeologists and psychological profilers.
A PSNI spokesman said the murder investigation was still live. “We intend to mount a series of searches for Arlene’s remains before the end of the year,” he said. “The investigation into her murder continues.”