THE disappearance last week from his home in Co Louth of the man gardai suspect ordered the Omagh bombing is being taken as an indication that the dissident republicans involved are beginning to realise the net is tightening around them.

The team of Garda investigators, led by Assistant Commissioner Kevin Carty, is understood now to have a full picture of the roles played by most of the individuals in the Omagh bombing. Republicans belonging to the groups called the "Real IRA", the Continuity IRA and the Irish National Liberation Army (INLA) were involved.

A key piece of information, relating to the use of mobile telephones minutes before and after the explosion, has also made gardai confident they will, finally, be able to bring the perpetrators to justice.

They have been able to establish that one of two mobile telephones seized in a house search in north Co Louth, near the Border, in December was used twice in Omagh just before and just after the explosion. They believe the telephone was used by the bomber to notify another man in south Armagh that the bomb had been planted and to issue a warning.

That warning included the erroneous information that the car bomb had been left outside the court house at the top of Main Street, causing the RUC to evacuate shoppers down towards the actual site of the explosion.

There was then another call minutes after the explosion, also from the Omagh area, but the point of this is not certain.

Gardai discovered that the telephones had been used by examining details of the transmissions through "cells", in this case a transmission aerial on the outskirts of Omagh.

The evidence adduced from these calls, along with other statement, eyewitness and circumstantial evidence, was considered strong enough to make some gardai want to move against the bombers before Christmas. Mr Carty and other senior officers sought advice from the Director of Public Prosecutions in December but it was decided to make further inquiries.

The evidence uncovered in December led to a turn-around in the investigation, which had begun to run down, with detectives from the original team returning to normal duties.

It also led to the increasingly confident predictions from the Garda Commissioner, Mr Pat Byrne, and the RUC Chief Constable, Sir Ronnie Flanagan, and from senior politicians, that the Omagh bombers would be caught.

Since December the investigation squad based in Monaghan and Carrickmacross Garda stations has been expanded back to its original 40 to 50 strength.

On Saturday more officers travelled from Dublin and other provincial stations to again assist in the investigation. There are now hopes that the bombers, those who supplied them with the explosives and car, and those who directed the bombing will be caught and charged.

Another boost to the investigation came last week when the Supreme Court ruled that the Garda i could be allowed to use the "right to silence" powers granted in the criminal justice legislation enacted by the Oireachtas after the Omagh atrocity. The legislation had been challenged through the High Court and was, until the Supreme Court judgment, in abeyance.

AS A result of the judgment gardai can now bring evidence in a court case that an accused refused to give details of his whereabouts when asked about specific offences. A court can "draw an inference" from this evidence, but other evidence must also be supplied to prove guilt.

Gardai believe the Omagh bombing plot began with a direction from a man who set up and directed the group which was referred to as the "Real IRA", whose political wing is known as the 32-County Sovereignty Movement, and which was based in the north Louth/south Armagh area. The group attracted other anti-ceasefire republican elements throughout the Republic and in south Armagh.

It is believed the materials for the explosives - ammonium nitrate fertiliser, sugar and diesel - for the Omagh bomb were gathered and mixed in farm buildings in Co Kildare and transported to south Armagh about two weeks before the August 15th bombing.

Two bombs were constructed by Provisional IRA bomb-makers from south Armagh who are opposed to the IRA ceasefire. One of these bombs was detonated, for testing purposes, in a field just yards north of the Border in the Cooley Peninsula above Omeath on August 6th, nine days before Omagh.

The bomb-makers planted flags at five-metre intervals from the test bomb to ascertain the extent of the blast. The bomb mixture was highly incendiary and the flash was seen over a wide area of the Co Down coastline along Carlingford Lough. The same incendiary mix contributed greatly to the appalling injuries from burns suffered by almost all of the 29 dead and 300 injured in Omagh.

According to one senior security source there are suspicions that the Omagh bombers actually designed their bombs as "anti-personnel" devices, intending to cause maximum collateral human injury rather than merely damaging property.

The bombers had also avoided detection by gardai, who had previously had a remarkable run of successes in stopping their bombs, by switching their bomb-making bases from the Republic to south Armagh.

The Omagh bomb was probably constructed in the Drumintee area and driven westwards by two local men, whose identities are now known to gardai and RUC. One of them carried the mobile phone with him.