The Bloody Sunday Inquiry, which resumes today, faces an imminent decision on British government submissions that secret intelligence material should not be publicly disclosed.
In what could be a landmark legal ruling, the tribunal judges must decide if the non-disclosure of this relevant evidence to lawyers for the Bloody Sunday victims, and to the public, is compatible with European Human Rights law and with their own mandate to carry out a fair and open inquiry.
Whatever the tribunal decides, its ruling may well be appealed by way of judicial review application to higher British courts, in the first instance, and possibly even to the European Court of Human Rights at a later stage.
The complex issues involved touch upon core concepts such as interpretation of "the public interest", whether this is synonymous with "Defence of the Realm" considerations, and how UK domestic and European Convention principles can be reconciled.
The practical bottom line for the tribunal, however, may hinge on its assessment of whether the course it adopts can command public confidence in the results of its investigation - an aspiration voiced by the British Prime Minister, Mr Tony Blair, when he set up the inquiry.
Secret intelligence documents and tapes dating from the 1980s are at the heart of the issue. These include allegations, said to have been made during the debriefing of a security service agent code named Infliction, to the effect that Mr Martin McGuinness of Sinn Fein admitted firing a shot which precipitated events on Bloody Sunday.
Heavily edited sections of these documents have already been exhibited at the public hearings in Derry into the killings of 13 men by British paratroopers during a Civil Rights march in the city on January 30th, 1972. The allegations have been dismissed by Mr McGuinness, who has not yet, however, provided a statement to the tribunal's solicitors.
The British government is now seeking to restrict the evidence shown publicly and wants the inquiry to screen security service personnel who may be called to testify.
Public Interest Immunity (PII) certificates have been submitted by the Home Secretary, Mr Jack Straw, and the Defence Secretary, Mr Geoff Hoon.
Mr Straw submits that full disclosure of the contents of security service documents and tapes could damage the work of the service. He also argues that disclosure could put security service agents, or former agents and informers at risk of reprisals from terrorist groups.
Lawyers for the Bloody Sunday victims' families have invoked European human rights legislation and principles in support of their argument that the sensitive information should be made known. But counsel for the Home Secretary contends that these same laws oblige the inquiry "to respect and protect the human rights of individuals who have assisted the security service".
The PII applications also cover documents related to a former security service officer, codenamed Observer B, and information relating to the identities of other officers identified only as A, C, David, James and Julian.
The families' lawyers point out that Article 2 of the European Convention on Human Rights, safeguarding the right to life, is now incorporated into UK domestic law. The application of its provisions, they argue, requires the inquiry to carry out a rigorous and effective investigation, particularly where deliberate lethal force is used by agents of the State.
They submit that to grant the PII applications would seriously damage public confidence in the tribunal and would demonstrate that special rules existed for the very state agents whose activities were being investigated.
In an outline written argument submitted on behalf of families represented by Madden and Finucane, solicitors, counsel commented: "We submit that it cannot be right that those suspected of the murder and attempted murder of those killed and wounded on Bloody Sunday should attempt to dictate what evidence will be made available to interested parties".
Counsel representing most of the soldiers who were on duty in Derry on Bloody Sunday has also urged full disclosure, on the basis that some soldiers might face serious criminal charges later and that all material which might help in their defence against allegations already made should be revealed.
Meanwhile, the tribunal will soon have to decide its next moves to secure co-operation with the inquiry by a number of key potential witnesses who had not responded to its requests for statements up to the time it adjourned in December. These include the unionist politician, Mr John Taylor, who is known to have chaired a crucial meeting of the Joint Security Committee at Stormont just three days before Bloody Sunday.
A Belfast solicitor, Mr Barra McGrory, made the first appearance for Mr Martin McGuinness at the inquiry on December 13th, when he applied for "full interested party status" for his client at the hearings.
The tribunal chairman, Lord Saville, noted that no reply had been received to six successive letters sent by tribunal lawyers to Mr McGuinness over the previous 18 months asking him to provide a statement. It was earlier pointed out that Mr McGuinness had been among those who had campaigned strongly for a new inquiry into the killings.
The chairman indicated the tribunal would be "extremely likely" to grant some form of legal representation to Mr McGuinness if he gave a full statement. But the tribunal ruled against the application for full party status for Mr McGuinness "unless and until he provides this tribunal with a statement of his recollection of, during and concerning the events of Bloody Sunday".
In recent sessions of the inquiry, it has become evident that an issue will be made by the tribunal and many of the participating lawyers of the identities of, and co-operation by, those who were members of the Official and Provisional IRA at the time of Bloody Sunday.
A few have already made statements and will be called to testify, but many others have not yet come forward and are still taking private legal advice on their options.
Since it began oral hearings last March, the inquiry has heard extensive submissions by the various legal teams and, up to the time of the Christmas adjournment, it had heard direct testimony from more than two dozen out of some 700 civilian witnesses. No military or police witnesses have yet given direct evidence.