Ten years after 29 people and two unborn babies were killed in the Omagh bombing, no-one is in jail for the attack.
Omagh was the biggest loss of life in a single attack in the Troubles
The BBC's Panorama programme has learnt that the UK's electronic intelligence agency GCHQ was monitoring exchanges between the bombers that day.
Here Panorama reporter John Ware explains why the GCHQ intercepts were not passed to detectives and how they may have helped.
Over 10 years there have been two police inquiries into the Omagh bombing costing many millions of pounds. The key suspects are well known. Why aren't any of them in jail?
The biggest complication facing Criminal Investigation Department (CID) officers in what was then the Royal Ulster Constabulary (RUC) was the fact that though the bombing was committed in their jurisdiction, 80% of the suspects lived in the Irish Republic, thus falling under the jurisdiction of the Republic's police force, the Garda Siochana.
The two police forces were governed by different rules and laws.
Despite frequent assurances from the UK and Irish governments that the police forces were co-operating to such an unprecedented extent that there was, in effect, a joint inquiry, in reality there were two separate and parallel inquiries.
An internal review by the RUC and later by the Police Ombudsman for Northern Ireland said there were serious flaws in the gathering and processing of evidence with leads not being followed up.
That being said, RUC detectives did manage to identify the mobiles that had been used in the planning and execution of the attack, and four other linked bombings, through "cell site" analysis of 6.4 million calls - which was widely regarded as a ground breaking piece of work.
However, although this proved which mobiles had been used and where (for example when the bomb car was being parked in Omagh) it did not prove who had been using them. Unknown to the detectives some of this evidence lay within the UK's electronic intelligence agency GCHQ, in the form of voice recordings obtained through interception, both by GCHQ and the Special Branch.
Why couldn't the intercepts be used as evidence in court against the bombers?
There has been a blanket ban on the use of intercepts as evidence in the UK.
In 2003 the then Prime Minister Tony Blair ordered a review to see if lifting the ban might secure more convictions of organised criminals and terrorists but the government concluded that "it was not the right time to change the law".
Intercept evidence is used in the courts of most other Western democracies such as Canada, Australia and the US, where it has put mafia mobsters behind bars.
Prime Minister Gordon Brown has now agreed to the limited use of intercepts being used as evidence, but intelligence agencies like GCHQ will still be allowed a veto. So in practice not much may change.
Sir Ronnie Flanagan, RUC Chief Constable at the time of the Omagh bombing, told the BBC he knew nothing about the GCHQ intercepts. Now Her Majesty's Chief Inspector of Constabulary, he says he has always been a supporter of lifting the ban.
Police officers or government officials are not supposed to disclose any details of an intercept.
But there was nothing in law to stop the details of GCHQ's intercepts from having been shared with the CID to help them develop evidential leads.
What difference would it have made if the intercepts had been shared with the CID?
Detectives talk about the "golden hours" in the immediate aftermath of a crime. This period offers the best opportunity of gathering evidence against criminals who may not yet have been able to dispose of weapons, clothing and equipment containing DNA, fibres and other forensic leads.
In the case of the Omagh bombing, had the mobile numbers used and the names of those believed to have been using them been disclosed immediately to the CID, the suspects' homes could have been raided and phones and clothes recovered for forensic testing, which may have linked them to cars used in the bombing.
Intercepts could also have helped the detectives carry out more effective interrogations. There is nothing more unsettling for a suspect than knowing that the interrogator knows the score; and nothing more undermining for the interrogator than when the suspect knows he is fishing.
Transcripts of interrogations with suspects show detectives casting their net wide. Without being briefed about what was in the transcripts, they had little to go on.
Some detectives believe that had they been entrusted with this information they might have secured some admissions, because emotions were so highly charged by the scale of the carnage.
Why was intelligence not shared with the detectives?
This remains the most perplexing question. Senior police sources believe the most likely explanation is a fear that details of GCHQ's secret technology and methodology would seep into the evidential chain if too much was shared with the CID and that this posed an unacceptable risk to the strategic capability of the UK's intelligence agencies.
It is also not clear who got what and when. One well informed source insists that the GCHQ material was available to the Special Branch within six hours of the bombing.
Special Branch sources have categorically denied this, insisting they were only given the details three to four days after the bombing.
Special Branch insist that they gave this information, which had been sanitised by GCHQ, verbally to the CID 24 hours later .
However police sources say, the CID log records that detectives did not get names until three and a half weeks after the bombing. But this was of little use to them because they got no other details on which they could build a case - not even the fact that the bombers had been using mobiles.
Special Branch sources say they were heavily restricted by GCHQ in what they could disclose to the detectives.
Is the failure to share intercept evidence the only reason no-one has been convicted?
Who can say?
Whilst the detectives have been criticised for not following up all of the leads, we will never know if they had, whether this would have led to charges.
The bombers were ex-members of the Provisional IRA, or Provos, who had left in protest at their leaders signing up to the Northern Ireland peace process.
Although Provo leaders condemned the bombing, they never encouraged their members who might have had evidence against the bombers to come forward with witness statements to the Garda Siochana or the RUC, now the Police Service of Northern Ireland (PSNI).
What happens now?
The Omagh families are suing five individuals who they say were responsible for the deaths of their relatives in the civil courts in Belfast.
The five, all of whom deny any involvement in the explosion, are: Michael McKevitt, currently in jail in the Irish Republic for directing terrorism; Liam Campbell; Seamus McKenna; Seamus Daly and Colm Murphy, who is also facing a retrial after his 2005 conviction for conspiring to cause the Omagh bomb was overturned.
Lawyers for the families say they will seek discovery of the GCHQ material through the courts.
They have renewed their calls for a public inquiry.
Panorama: Omagh - What the Police Were Never Told will be broadcast on BBC One at 8.30pm on Monday 15 September.