Sinn Fein is backing calls for a cross-border independent inquiry into the police probe following the Omagh bomb.
Deputy First Minister Martin McGuinness said there were serious concerns about the "debacle after debacle" which plagued security force inquiries into the conflict's bloodiest atrocity.
He was speaking after a service on the 10th anniversary of the 1998 Real IRA car bombing which killed 29 people and unborn twins.
Nobody has been convicted of causing the deaths.
"The calls that are made from the families here for the establishment of an independent tribunal, they obviously have lost all faith in the police investigation and we have seen debacle after debacle," he said.
"I think what we need to do is support the families in the demands that they are now making."
Families have voiced concern about alleged police failure to act quickly enough on intelligence warnings of the planned bombing.
There have also been a string of flaws in forensics handling of the investigation, highlighted in the trial of Sean Hoey, of South Armagh, who was acquitted of murder at Omagh.
The families have secured support from other political parties in their quest for justice.
Mr McGuinness added: "This was a very clear attempt by whoever was behind the bomb to destroy the peace process, destroy Sinn Fein's peace strategy. Ten years on they have failed miserably."
