Relatives of the Omagh bomb victims are to sue Northern Ireland's police chief for failings they believe allowed the killers escape justice.
Twenty-nine people - including a woman pregnant with twins - were killed in the Real IRA attack in 1998.
A writ has been issued against the chief constable, focussing on what happened after the car bomb and why no-one has been convicted of murder.
The Police Service of NI (PSNI) said they "would respond in due course".
The writ is the latest legal challenge in the families' two-decade quest for justice.
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.
"Here we are 19 years on and the criminals responsible for this are still walking the streets - there has been absolutely no punitive measures taken against any of them," he said.
"We can't walk away and say 'it's just one of those things'."
Stanley McComb, whose wife, Ann, was killed, said there was "no way" he was giving up.
"Every morning I waken I am on my own, every night I go to bed I am on my own," he said.
"My partner, my wife of 25 years, is no longer there and should be there. It does hurt you, you learn to live with it, but that's what drives me on."
The relatives have already successfully sued four republicans in a landmark civil trial that found them liable for the bombing.
Baroness Nuala O'Loan, former Northern Ireland police ombudsman, told BBC Radio 5 Live the investigation was "very flawed".
"What we found when we investigated the investigation was that there were many, many, many stones which were left unturned," she said.
"There was a failure to share intelligence with the officers who were investigating the crime.
"It was a very, very poor investigation."
But Baroness O'Loan said it was impossible to tell whether alleged failings had harmed the inquiry.
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.
The latest action is issued in Mr Gallagher's name on behalf of the families belonging to the Omagh Support and Self Help Group. They are seeking damages and a declaration that their human rights were breached.
It has been issued against PSNI Chief Constable George Hamilton because he has legal responsibility for the actions of both his service and its predecessor, the Royal Ulster Constabulary (RUC).
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.