Revelations that the security services were recording the Real IRA plotters’ phone calls on the way to Omagh in 1998 left them with no other option, family members told a meeting in Belfast.
Twenty-nine people, plus unborn twins, died in the worst atrocity of the Northern Ireland conflict.
Michael Gallagher said there could be criminal proceedings for perverting the course of justice.
He said: “We believe the intelligence services, the police service and those who work in law and order have responsibility and have to live up to that responsibility, and there has to be some degree of accountability.
“Until we do have that, we will have an intelligence service that is possibly out of control and a law unto itself.”
Mr Gallagher was joined by about a dozen other relatives, including Carol Radford and Stanley McComb.
Ms Radford said: “The whole point is to try to put the bad guys away — what is the point of gathering it [intelligence] if you are not going to use it?
“The government can’t walk away from this.”
Mr Gallagher said he would be writing to Police Service of Northern Ireland chief constable Hugh Orde and had also sent letters to the Irish and Spanish governments, whose citizens were killed.
“We feel that there may be a criminal act committed here, there could be the perversion of justice, there is a charge of withholding information about a serious crime,” he added.
Families have demanded an independent public inquiry on both sides of the Irish border to probe the failure to intercept the killers or put anybody behind bars.
Former police ombudsman Nuala O’Loan has published a damning critique of the police investigation, accusing detectives of letting down victims and their families.
Mr Gallagher said the BBC Panorama revelations left families in a similar state of shock.
“After 10 long years our questions cry out for answers. The issues raised are the meat of public inquiries both north and south of the border. The authorities should give us more immediate help.”
Mr Orde has admitted it is unlikely the killers will be brought to justice.
Colm Murphy was sentenced to 14 years in jail in the Republic for conspiracy to cause the explosion but the conviction was later overturned and he is awaiting a retrial.
The only person charged directly with the murders, south Armagh man Sean Hoey, was cleared of all charges last December.