Relatives of victims of the Omagh bomb are suing the Chief Constable because of "spectacular failings" in the criminal investigation, the father of a young man murdered in the atrocity has announced.
Twenty nine people, including a woman pregnant with twins, died and more than 220 people were injured when a Real IRA car bomb exploded in the Co Tyrone town on August 15, 1998.
A writ was issued yesterday against PSNI Chief Constable George Hamilton.
The families said their decision followed failings in the investigation and a failure to secure convictions.
The relatives are pursuing judicial review proceedings against the Government's decision not to hold a public inquiry into claims the attack could have been prevented if it had not been for a series of intelligence errors.
Ahead of the 19th anniversary of the atrocity, which made headlines around the world, Michael Gallagher, who lost his 21-year-old son, Aiden, in the bomb, said families were pursuing legal action out of desperation.
"This writ has come out of sheer frustration," he told the Belfast Telegraph.
"This was the worst crimes of the Troubles and we believe that there has been missed opportunities.
"This was a crime that world leaders took an interest in, but yet it wasn't enough to bring one conviction.
"We see people responsible for this crime walking the streets and not a single punitive measure taken against them.
"This was a crime that happened 19 years ago and we want to know why those responsible were not brought before the courts with good evidence and convicted.
"They probably would have been out of jail by now.
"That failure is not just on the part of the Chief Constable. The Garda have had no success either."
Mr Gallagher also stressed that he fully supported the police and said he hoped there would be officers at the memorial service on Sunday.
"We just feel that, 19 years on, we haven't seen any justice - we need answers," he added.
"It is an act of desperation. Now is the time to get answers.
"We need to know why they have failed so spectacularly on the worst atrocity of the Troubles, even though the atrocity was in peacetime.
"What we want is for the people behind this crime to be put in jail."
Asked about the writ, a spokesperson for the PSNI said: "Once received, we will take time to consider the contents and respond in due course."
The relatives previously successfully sued four republicans in a landmark civil trial that found they were responsible for the bombing.
The latest court action has been issued against Mr Hamilton because he has legal responsibility for the actions of both his service and its predecessor, the RUC.
The action is issued in Mr Gallagher's name on behalf of bereaved families belonging to the Omagh Support and Self Help Group.