There is still a distinct whiff of cordite in the Derry air in the wake of the devastating (though, in part, dubious) challenge delivered, in his absence, to Martin McGuinness during the Bloody Sunday Inquiry on Thursday.

By a bizarre coincidence, it came just hours after the city was jolted awake by the dawn explosion of a bomb placed at the perimeter of the large British army base beside the River Foyle.

As the echoes of this detonation revived long-dormant feelings of foreboding and menace among a community once almost inured to daily turmoil, the inquiry and those observing it were rocked by the legal Exocet aimed at Mr McGuinness.

It has prompted an urgent reassessment of the implications and contemporary role of the tribunal, and of the style and mettle of its leading counsel, Christopher Clarke QC.

It is evident that the inquiry, as it begins to unfold, cannot be seen solely as a detached academic exercise, engaged in historical research to "set the record straight".

Like it or not, it is very definitely inter-linked with, and part of the contemporary political dynamic.

It may have pretty teeth, like the proverbial shark, and it comes clothed in Saville Row suits, but it can and will draw blood.

The introduction of British Secret Service material, of a provocative nature, anonymous and obviously selective, carries with it the bitter-almond foretaste of cyanide.

Plainly, once these documents came into the possession of the tribunal (and we have yet to hear an elucidation of this process), they had to be addressed. But the manner and timing of their opening raises questions.

The measure of Mr Clarke's professionalism and impartiality may be taken by the degree of rigour and scientific scepticism which he brings to bear in dissecting this material, probing its provenance and authenticity.

It is, at best, hearsay material. True, we have already heard an abundance of hearsay evidence in the past two weeks, much of it fascinating and potentially highly relevant. There will be more. The same standards of legal assay must be applied to it all if the inquiry is to maintain credibility.

Yet there can be no quibble with Mr Clarke's eloquent case that Mr McGuinness should offer himself as a witness. His reasoning was compelling and comprehensive. It lacked just one ingredient - a cast-iron, predetermined guarantee of future outcome.

By definition, an inquiry cannot supply that. It can demonstrate, over time, convincing and impressive method in action. But how long can Mr McGuinness, or the tribunal, or the Bloody Sunday families, wait?

Some relatives of the victims, including Sinn Fein supporters, have urged former IRA members, in unambiguous terms, to come forward and co-operate with the inquiry.

But it is widely reputed (to use a Clarke term) that certain key figures in the day's events are still in the throes of a moral, ethical, legal and even political dilemma over their options.

With or without them, the inquiry will go ahead, but the picture will be incomplete and the outcome may be distorted. There are certain striking parallels with events in the wider Northern political process.

Meanwhile, Mr Clarke's tour de force through the evidence continues to churn up a welter of previously buried facts, partial facts, contemporary comments, impressions and rumours, leaving in its wake a growing list of new questions, side-issues and byways that invite exploration.

It seems, at times, like a staged re-opening of Pandora's box.

Take, for example, the highly controversial series of detailed typewritten notes, recovered from Sunday Times archives and said to be contemporaneous journalistic records of conversations with figures such as Ivan Cooper and Bernadette Devlin (there may be others).

Their vivid and almost scandalously candid nature has provoked, and will continue to provoke, near-apoplexy among some senior figures. Mr Cooper has termed them "poisonous" and they will be severely tested when the lawyers are unleashed.

The notes, now part of the documentary record held by the tribunal and circulated to participating parties, contain an allegation that the Provisional IRA at one stage considered assassinating the then head of the RUC in Derry, Chief Supt Frank Lagan, a Catholic.

" . . . there was a plot to shoot Lagan as he took his child to St Columb's College. IC [Ivan Cooper] got wind of it (via McEvoy presumably) and persuaded them not to kill him, largely on the grounds that he was their only contact on internees etc. IC also told Lagan not to take his kid to school again. That was a month before Bloody Sunday."

There is more, some of it very serious and some commanding interest only on the level of political gossip.

This material, never published and obviously not intended for publication - at least in its existing form - will nevertheless be trawled through, it seems, in the oral examination of Mr Cooper and Mr McGuinness and "such journalists from the Sunday Times as may have been responsible for this document".

This week's proceedings at the inquiry have certainly made the issue of participation more problematic for Sinn Fein and IRA figures. It should be remembered that they are not alone.

The same dilemma has faced or is facing soldiers, officers, civilians and also political figures up to the highest level.

The British Attorney General has given an undertaking that no evidence or written material provided by a witness "will be used to the prejudice of that person in any criminal proceedings against that person, or for the purpose of investigating or deciding whether to bring such proceedings, except proceedings where he or she is charged with having given false evidence . . ."

Yet there is still, in many quarters, deep reluctance to come forward. The tribunal's legal officers refuse to provide any details of individuals who have been invited to supply statements and have not yet done so.

However, those who observe the proceedings may note the lack of references, so far, to witness statements from certain prominent figures.

But these names, like that of Mr McGuinness, will inevitably arise, and they too will be faced with a public challenge.

It is still early days, but a sense is emerging of the immense statutory powers which this inquiry commands and seems determined to employ.