Republicans are prepared to co-operate with any independent, international inquiry into the Omagh bomb atrocity, they confirmed tonight.
Martin McGuinness, the Sinn Féin chief negotiator and Deputy First Minister at Stormont, made the pledge as victims relatives intensified their campaign for a cross-border probe.
The assistance would be to help expose alleged Police Service of Northern Ireland incompetence and claims that officers knew of the attack in advance, Mr McGuinness claimed.
He said: "Republicans would be only too glad to co-operate with any independent, international investigation into the bomb explosion, because we think the PSNI themselves have questions to answer.
"There's a very strong belief within Irish Republicanism that the PSNI not alone failed to investigate the Omagh bomb properly, but the RUC actually knew about the bomb before it took place."
The families of some of the 29 people murdered in the dissident Real IRA massacre viewed Mr McGuinness's declaration as a significant advancement on Sinn Fein's previous position on the bombing.
Michael Gallagher, who lost his son Aiden in the August 1998 attack, said: "This is progress and something we welcome.
"It was unexpected that he said that, and we would be interested to hear now how Sinn Féin and the Deputy First Minister are going to move this forward."
The families' demands for a full independent inquiry into Omagh, a case plagued by controversy over what intelligence police possessed and passed on to officers on the ground, comes amid their protracted civil action against five men suspected of involvement in the bombing.
The multi-million pound case which is due to be heard at the High Court in Belfast has been hit by further delays.
Only one man has been charged with the Omagh murders, south Armagh electrician Sean Hoey.
A judge is due to deliver his verdict later this year after studying masses of evidence presented during Hoey's trial in Belfast.