The FBI has forbidden an agent who infiltrated the Real IRA from giving evidence at a civil action being brought by relatives of the Omagh victims, a Belfast court heard yesterday.
Families for six of the 29 people killed in the 1998 atrocity wish to submit transcripts of evidence from spy, David Rupert, as part of their landmark case.
Mr Rupert, who is now in a witness protection programme, testified against Real IRA godfather, Michael McKevitt, during his trial in Dublin in 2003.
His evidence was regarded as valuable in securing a guilty verdict and McKevitt is now serving a 20 year term in Portlaoise Prison.
The families are suing five men - Michael McKevitt, Liam Campbell, Colm Murphy, Seamus Daly and Seamus McKenna - for £14 million exemplary damages. All five deny any part in the mass murder.
During day six of the multi- million pound civil case a barrister acting for McKevitt said he understood that the FBI had simply "vetoed" any appearance by Rupert in Belfast.
Submissions on Rupert's evidence are due to be heard next week and if Mr Justice Morgan admits the series of emails then the Michael O'Higgins SC said he would be seeking an explanation as to why Rupert was not appearing in court.
Mr O'Higgins then said the material would be trawled for deficiencies which he claimed were "many and varied".
He said: "When we deal with admissibility, one of the issues we deal with is why Mr Rupert isn't here. I mean in the wider sense - on the television and not necessarily in the witness box.
"The court needs an explanation. We do not regard an explanation from the well of the court as to why Mr Rupert isn't in the court.
"Our understanding is that the court made an order that his evidence could be given by video- link. That appears to have been abandoned in the opening. We don't necessarily know the reasoning for that but our understanding is the FBI simply vetoed it."
If Rupert's evidence is admitted then Mr O'Higgins said the defendants' legal teams would want "live witnesses" who could explain his absence and who could be cross-examined.
Meanwhile, earlier yesterday a telecommunications expert told the court that the probe into the Omagh bomb had been the biggest investigation in which he had ever been involved.
Raymond Green, a former fraud and crime investigator with mobile phone giant Vodafone said his staff spent "two or three months" trawling the records of four mobile phone numbers given to them by the RUC.
The cell site analysis was given to the police on phones identified in court as belonging to people named Murphy, Morgan, Treanor and Brady, and released under the Data Protection Act.
Mr Green, who worked with Vodafone for nine years, described his efforts to assist the RUC and Gardai as "very significant".
He told the court that in November 1998 the RUC requested information on calls made to and from four mobile phones, including dates, times and the duration of the calls made from the phones.
None of the families were in court yesterday.
The civil action is regarded as their final hope for justice.