The incident claimed the lives of 29 people in 1998
Relatives of the Omagh bomb victims are to sue the PSNI Chief Constable George Hamilton for investigative failings they believe let the killers escape justice.
The Real IRA incident claimed the lives of 29 people on August 15th 1998.
Fifteen of the dead were under the age of 21, while one woman was pregnant with twins.
Families say they "need answers" and claim the attack could have been prevented.
They have issued a writ against Mr Hamilton seeking damages and a declaration their human rights have been breached.
Murder charges against the only remaining Omagh bombing suspect were dropped by prosecutors back in March last year.
An investigation by the Police Ombudsman of Northern Ireland back in 2015 found Special Branch did not provide all of the information they had to detectives investigating the bombing.
The probe showed intelligence officials withheld details, like phone numbers, for security reasons.
But it concluded that this would not have prevented the attack.