Omagh bomb victims' relatives are due to meet Gardaí to voice concerns about the investigation into the man alleged to have stolen the car used in the atrocity.
The families will also seek assurances detectives are co-operating fully with their lawyers in a multi-million pound civil action against five dissident republicans.
Mr Michael Gallagher, who son Aidan was among the 29 people and two unborn babies killed in the August 1998 "Real IRA" attack, warned tough questions would be asked.
"This is our first briefing with the southern investigating team and it's long overdue," he said. "We want to know about the person identified who is alleged to have stolen the Omagh bomb car and why he was never charged with providing what is in effect the murder weapon".
Mr Gallagher said Assistant Garda Commissioner Kevin Carty will be challenged by relatives during the meeting in Co Monaghan to produce evidence of progress.
"We still haven't seen one person charged with murder in Omagh," he said.
The families are trying to raise enough cash for their High Court action later this year against the men they claim plotted the attack.
But Mr Gallagher claimed the investigators may have been able to provide more help. "We have concerns also about the lack of co-operation between gardaí and our legal team which we need to raise," he said.