To the inexpert eye, the early forensic work seemed anything but slapdash. The white- suited figures picking their way in the rain through the rubble of Omagh's narrow main street had the air of people studying an archaeological site, trying to find a pattern in apparent chaos. So it seemed.

Sometimes shattered glass crunched beneath their feet. Now and then the broken bones of a building would settle and groan. In Northern Ireland, nevertheless, there was no shortage of expertise for the sort of work that was being conducted with such care. So it seemed.

Among the media behind the police lines there was no shortage of agreement, either. It was already known that this was the biggest, bloodiest terrorist "spectacular" of them all. No effort would be spared to find the perpetrators. Nor, this time, was there the whisper of a word to excuse, explain or defend them. The self-validating conspiracy theories - the Brits did it - would be applied retrospectively.

Omagh is hard to describe. I don't mean the brute facts: the 29 dead, nine children and a woman pregnant with twins among them, or the hundreds wounded in body and mind in a country town with no sectarian history. I don't mean the physics, either, the way in which materials react to blast and heat to make, sometimes, a bizarre sort of found art.

Omagh was numbing. The shock wave travelled far and left no-one untouched. It went deep, too. This is hard to say without appearing to diminish 3000 and more other deaths, but Omagh horrified a province that was, supposedly, inured to horror. It was (and who invented this scale?) beyond what could be endured. Anyone who regarded it as just another "mis-calculated" act of war lacked the nerve to say so.

That, however, was another reason to believe that Omagh's killers would be found quickly. A peace of sorts had come to the north, but it was a new thing, and precarious. Clearly, the atrocity was intended to return the province to war. So who - the elementary question - had the motive? Not many.

Some people would later nominate agents of the British state to explain why two warnings were, apparently, ignored. Abundant evidence of disgraceful security service behaviour in other cases - up to and including contracted murders - was employed to describe a plot intended to discredit Sinn Fein and an IRA cease-fire. But nominated assassinations and indiscriminate mass murder are very different things.

Irish republicanism's history of ideals betrayed and revenges exacted was always a better place to start. The self-styled Real IRA existed, such as it was, to "continue the struggle". Gerry Adams and Martin McGuinness had settled for negotiations, for less than the old dream. Omagh's point was to make their task impossible, their strategy unworkable. Instead, it did the opposite. Peace held, was strengthened. Even rogue MI5 agents might have seen that coming.

It should, in any case, have simplified police efforts on both sides of the border. The group behind Omagh was a small one, self-defining and, in intelligence terms, self-advertising. Evidence liable to stand up in court might be hard to come by. Terrorism, as a government pursuing indefinite detention likes to remind us, is like that. Nevertheless, this was an area in which the Royal Ulster Constabulary and the Gardai had a deal of unwelcome experience.

Reading the conclusions of Mr Justice Weir, sitting alone in a Diplock court almost nine and a half years after the bombing, and almost a year after the end of the three-month trial of Sean Gerard Hoey, you could almost begin to believe the conspiracy theorists. Carelessness, stupidity, incompetence, dilatory leadership, arrogance, complacency, tainted DNA evidence, lying coppers: the judge in Belfast was left with no choice. He had no choice but to acquit Hoey, nine years after the electrician became a suspect, and he had no choice but to damn the police. Their efforts were, in every sense, too bad to be true.

This had been pointed out before. Ombudsman Nuala O'Loan was treated to a campaign of vicious denigration - and in the case of Peter Mandelson, silky Stormont contempt - when she reached much the same conclusions as Judge Reg Weir. Sir Ronnie Flanagan, then head of policing in Northern Ireland, even said he would commit suicide if he once thought O'Loan's documented allegations were correct. Instead, Sir Ronnie is these days the Chief Inspector of Constabulary. His position is in some doubt at the time (about time) of writing, but he remains hale and hearty.

It is possible, even likely, that Flanagan takes the view adopted by the Met's Sir Ian Blair after Jean Charles De Menezes was shot in the head at Stockwell Tube station in 2005. How reasonable is it to expect one individual to accept personal responsibility for the actions - or inaction - of so many subordinates? It is reasonable, apparently, to accept a knighthood for their work in the better times.

But terrorism manifests itself with a mixture of brutal simplicity and fiendish complexity. Split-second decisions are required, fine judgments are involved, wider issues are always at stake. And "terrible mistakes" - a warning overlooked, an innocent man misidentified - do happen. Good enough?

Actually, conceivably, yes. Sometimes, when the truth, all of it, has been told, those answers might suffice. The state requires its servants to combat those who put our common security at risk. We could argue over motives, perspectives, tactics, ideologies, resources, even conspiracies, but the possibility of error, pure or otherwise, is undeniable. As is the possibility that important men sometimes evade both truth and responsibility.

Why is Blair still in his post after his blatant attempts to delay the Independent Police Complaints Commission (IPCC) investigation - a statutory obligation - into the De Menezes shooting? Why is Flanagan allowed to inspect constabulary, a joke in very poor taste, when the government is contributing £800,000 to the "exceptional case" private prosecution being mounted by the Omagh families? Disparate cases do not afford generalised conclusions, whatever the conspiracy theorists might believe, but some patterns are impossible to ignore.

The IPCC will not now be taking action against four Met officers for their roles in the killing of De Menezes. As with Omagh, no-one is being held to account. Combine crude with legal, and you could say this: no-one did it. Yet, in the matter of Lockerbie, the biggest atrocity of all in these islands, we are offered the surreal converse: officially, one man did it. Except, of course, he did not.

Abdelbaset Ali Mohmed al Megrahi will be plucked from Scottish justice, and from his belated exoneration, in a deal whose existence was denied by our former Prime Minister while the Crown, citing a public interest it will not define, withholds a key document from a Scottish court. The document might demonstrate a miscarriage of justice. A reasonable person will ask, such being the case, why the Crown has elected to obstruct a court.

The truth is known to some, but not granted to all. On what authority? In each of these cases, judges have proved impotent and justice has been denied, blatantly, shamelessly. A pattern does not prove a theory. It certainly does not prove conspiracy. But this secret Britain has begun to stink, and stink badly.