READ MORE- Real IRA bombers found guilty in Omagh bombing civil case

SEE PHOTO GALLERY- Omagh bombing, County Tyrone, 1998

An FBI informer who played a crucial role in the case of a IRA leader found liable for the Omagh bombing is a pathological liar, an appeal court heard yesterday.

Undercover informer David Rupert, gave evidence in the civil action against Michael McKevitt. He infiltrated Real IRA support groups in the United Stats and then wen  to Ireland where he became a full time spy for the FBI and others

McKevitt, Seamus Daly, Liam Campbell and Colm Murphy, were found responsible for the bombings in a landmark civil case taken by the victims relatives in 2009.

The four men began their appeal against the judgment in Belfast High Court on Monday.

The Co.Tyrone bombing killed 29 people, including a woman who was pregnant with unborn twins, on August 15, 1998. A further 200 people were injured when the car bomb exploded in the town center on the Saturday afternoon.

The FBI official did not give evidence at the civil trial in Belfast, but testimony he has previously given against McKevitt at his 2003 criminal trial was submitted to the High Court.

During his opening remarks, McKevitt’s barrister, Michael O’Higgins, told the court objective evidence showed Rupert was a liar.

"Mr Rupert is a pathological liar and a confidence trickster, and a man who it was very strongly submitted (at civil case), a submission based on forensic investigation, engaged in serial perjury in the course of giving his evidence in the Dublin trial," he said.

Mr O’Higgins said that because Rupert has not given evidence in the civil case, McKevitt was denied the opportunity to cross examine.

To date no one has been successfully convicted for the Omagh bombing. Sean Hoey from south Armagh was acquitted of the criminal charges brought against him in 2007.

The families multi-million civil action was described by them as an attempt to bring as much information into the public domain as possible.

Stanely McComb, whose wife Ann was one of the 29 victims who died, was present in court yesterday to represent the victims.

"It's been a long 12 years for us, of listening to what's been said and what's been done," he told the BBC.

"The names of people crop up in it that you don't want to hear their names because it does make you angry. They are portrayed as being the good guys by their legal teams,” he added.

READ MORE- Real IRA bombers found guilty in Omagh bombing civil case

SEE PHOTO GALLERY- Omagh bombing, County Tyrone, 1998