Omagh was the biggest loss of life in a single attack in the Troubles
Families bereaved by the Omagh bomb have threatened legal action to try to force the intelligence services to hand over information about the bombing.
The move follows an investigation by the BBC's Panorama programme.
It revealed that GCHQ was monitoring Real IRA phone conversations as the bombers drove the device into the town in August 1998.
The families want access to tape recordings of the conversations and/or transcripts of what was said.
The relatives are currently involved in a civil action against those they believe they were involved in the bombing and want the phone evidence to try to help their case.
If the information is not voluntarily handed over in the next week, the families say they will take legal action to try to make it happen.
Bertha McDougall, the current chair of the Commission for Victims and Survivors, said the victims' families have unanswered questions.
"At this stage the commission has an open mind on whether there should be a public inquiry or not.
"We intend to form a view on that matter by meeting with families and by urgently meeting with the secretary of state and the justice minister from the Republic," she said.
Twenty-nine people and unborn twins were killed in the attack, which was the worst single atrocity of the Troubles, in Northern Ireland.