A prosecution over the Omagh bombing could have happened if there had been a better police investigation, a former police ombudsman has said.
Twenty-nine people, including a woman pregnant with twins, were killed in the Real IRA attack in 1998.
Relatives are to sue Northern Ireland's police chief for failings they believe allowed the killers to escape justice.
A writ against the chief constable looks at events after the bomb and why no-one has been convicted of murder.
In 2001, the then police ombudsman Baroness Nuala O'Loan published a report into the Omagh bombing that was highly critical of the police.
Speaking to BBC Radio Ulster on Friday, she said: "I think there's a distinct possibility that better police investigation, informed by intelligence, might have led to a prosecution and even a conviction.
"When I looked at the intelligence picture behind Omagh and then looked at how much had been given to the investigators, I found that only about a fifth of the intelligence which was available had been handed over.
"You have to bear in mind that the senior investigating officer didn't get specific video footage and things that he'd asked for, so there was a huge amount that just was not done in the Omagh investigation.
"I think some of the officers involved tried really hard, it [the investigation] just could not achieve what it was set up to achieve and I think it's terribly cruel that this had to go on."
Omagh bomb timeline
- 15 August 1998 - A large car bomb explodes on a Saturday afternoon in the centre of Omagh, County Tyrone, fatally wounding 29 people.
- 18 August 1998 - The Real IRA claims responsibility for the bomb.
- 6 August 2003 - Alleged founder and leader of the Real IRA Michael McKevitt is found guilty of directing terrorism.
She added: "I have seen evidence of decisions made in RUC Special Branch which are inexplicable
"I have seen ongoing failure to deliver, if you like, right from the beginning, to deliver the resources and to deliver the commitment and to enable the truth to be told."
The chief constable of the Police Service of NI at the time of Mrs O'Loan's 2001 report described her findings as neither a "fair, thorough or rigorous investigation".
"I consider it to be a report of an erroneous conclusion reached in advance and then a desperate attempt to find anything that might happen to fit in with that, and a determination to exclude anything which does not fit that erroneous conclusion," Sir Ronnie Flanagan said.
Quest for justice
The families' writ is the latest legal challenge in their two-decade quest for justice.
The PSNI said it "would respond in due course".
The Omagh bombing inflicted the greatest, single loss of life of any terror atrocity in the history of the Northern Ireland Troubles.
Michael Gallagher, whose 21-year-old son, Aiden, died in the bomb, said they needed answers.
Stanley McComb, whose wife, Ann, was killed, said there was "no way" he was giving up.
The relatives have already successfully sued four republicans in a landmark civil trial that found them liable for the bombing.
Police ombudsman reports have raised serious concerns about the police investigation.
They said evidential opportunities were missed, that intelligence information was not shared and there were inexplicable delays in arresting suspects
A PSNI spokesman said of the writ: "Once received, we will take time to consider the contents and respond in due course."
The families are also pursuing judicial review proceedings against the government's decision not to hold a public inquiry into the attack.