There's further controversy ahead on secret information withheld from the police investigation into the Omagh Bomb atrocity.

The BBC has discovered that the UK's electronic intelligence agency 'GCHQ' recorded mobile phone exchanges between the Omagh bomb team members on the day of the attack.

Tonight's Panorama programme is to claim that the calls were monitored as the bombers drove the car bomb into the Co Tyrone town.

After seeing the programme previews, bereaved relatives are already calling for a full public inquiry into what was the worst single atrocity of the Troubles, which killed 29 people and unborn twins.

Despite police inquiries on both sides of the Irish border over the last 10 years, at the cost of tens of millions of pounds, none of the bombers are in jail.

GCHQ was monitoring the bombers' phones that day, but whether GCHQ could have helped stop the bombing comes down to whether they were listening to live exchanges between the bombers, allowing them to respond to events, or whether they were simply recording the conversations.

It appears that, some weeks before Omagh was attacked, the Royal Ulster Constabulary (RUC) Special Branch was given a mobile phone number being used by bombers operating mainly from the Irish Republic. That number was passed to GCHQ for monitoring.

Phone billing records show the Omagh bomb run began in Castleblaney in the Irish Republic at around 12.40 on 15 August, with two mobiles having a 14-second exchange - the first of nine such exchanges both before and after the bombing.

One mobile was in the scout car which was checking the road ahead was clear, the other in the bomb car.

As Panorama reports, if intelligence officers were listening 'live' there were clues in the conversations, which though coded, could have acted as warnings.

After the bombing, Panorama says, Special Branch asked GCHQ what happened and was told: "We missed it."

Whether "missed it" was because GCHQ was simply recording the conversations, or whether officers had been listening in but had not understood the significance of the coded fragments, is not clear.

But, as Panorama reports, even if GCHQ could not have prevented the attack, more could have been done to help the investigation.

According to one of the sources who spoke to the programme, transcripts reporting exchanges with up to five mobiles associated with the bombers were sent to Belfast "within hours" of the bombing.

However, these were never disclosed to the detectives hunting the bombers.

In fact, Sir Ronnie Flanagan, former Chief Constable of both the Police Service of Northern Ireland and the RUC, told Panorama he was unaware GCHQ had been monitoring the bombers' mobile phones.

(BMcC)