The use of the Czechoslovakian explosive Semtex in attacks on the police was once common. It is less so now. In fact, it is not meant to happen. Nor does it, as a rule. After Colonel Gaddafi supplied large consignments to the IRA in 1986 and 1987, the manufacturers tightened their security.

It became much more difficult for illegitimate users to obtain the powerful explosive and the formula was altered to make it easier for police and Army dogs to detect it.

But Gaddafi's shipments to the IRA were large. When the Eksund was intercepted by French Customs in October 1987 with 150 tons of arms aboard, it was found the cargo included two tons of Semtex. The IRA is believed to have received three other consignments from the same source which were successfully smuggled in. It is believed to have had nearly three tons in its dumps at the time decommissioning was announced in 2005.

It is almost three years now since General John de Chastelain told a news conference in the Culloden Hotel in Co Down that the arms put beyond use by the IRA included "explosives and explosive substances". "We have determined," he said in the report of the International Commission on Decommissioning, "that the IRA has met its commitment to put all its arms beyond use ..."

His report was warmly welcomed by the Government and by the Irish government. The decommissioning was described as massive. It was said to mark "the end of the IRA's killing machine". The two clergy who acted as witnesses to the undercover process, one a Catholic priest, the other a Methodist minister, said they were "utterly certain" that the IRA had fully decommissioned. But it was freely admitted that there was an element of trust involved.

The role of doubting Thomas was played by the DUP leader, Ian Paisley.

He wanted the amount of arms quantified. He wanted to know how they were decommissioned; and he accused ministers of duplicity and dishonesty and a cover-up.

His scepticism was shared by Sir Reg Empey, leader of the UUP, who said: "Unionists remain to be convinced of the republican movement's commitment to exclusively peaceful and democratic means."

This last phraseology is interesting, for the leader of the UUP did not refer to the IRA, but to "the republican movement". His careful distinction underlines the crux of the present difficulty; for it is no secret that there are elements of the same movement which are anything but dedicated to the use of only peaceful means in furtherance of their objectives. What is not known is the extent and nature of the contacts which exist between those elements and the IRA proper — of which they used to be part. Everyone knows the links exist in a movement which has its amorphous arms. We live in a tribal society. Fifteen months ago in Londonderry Magistrates Court, a man faced seven charges linked to the alleged kidnap and assault of a man and woman in the Republic. The victims were subsequently found in the early hours of the morning, battered and bleeding, in a street in Creggan, the man shot in the lower legs. Was this political — or a smugglers' feud? It is difficult to say. But the individual charged had a crowd of supporters outside the court, whom he greeted with a clenched-fist salute and the episode had the stamp of a republican sanction. Whatever the truth, the Deputy First Minister, Mr McGuinness, in condemning the outrage, confirmed that the man in the dock was his brother-in-law.

Such connecting links comprise a subterranean wavy line into which the International Commission was ill-equipped to inquire. It merely pronounced that the decommissioning comprised "the totality of the IRA's arsenal". What is being asked once more, now that a republican gang has used Semtex to try to murder policemen in Fermanagh, is — how did the members of the Commission know? They spent a week of long days being ferried round Ireland in a van with blacked-out windows, much of their time being spent in Cork, Kerry and Tipperary. There they saw guns being lifted from concrete bunkers on isolated farms.

All this prepared the way for the IRA's about-turn on the use of violence. But it was essential to build trust across the wider community if the renunciation of violence was to bear the weight of a shared Government. That is why the Semtex in Fermanagh matters.

It has dealt a body blow to what trust there was. How much more of the republicans' ‘decommissioned’ armament is still on active service?