The provision of a statement last week by a former member of the Army's agent-handling Force Research Unit leads the Smithwick Tribunal into a forbidding area.
While the statement, by Ian Hurst, is for 'investigatory' purposes only and cannot be used as evidence, it nevertheless leads the tribunal's lawyers towards documents which might in themselves become evidence exhibits.
They will become evidence if they show a link between IRA members operating along the border and any Garda officer they compromised, or had contact with, in the period before the murders of Chief Superintendent Harry Breen and Superintendent Bob Buchanan.
The March 1989 ambush by the IRA followed a meeting arranged just hours earlier between Breen and Buchanan and their counterparts at Dundalk Garda station.
In spite a threat hanging over the tribunal by the Republic's justice minister, Alan Shatter, that it could be wound up by November, concluded or not, little has emerged to date that throws new light on alleged collusion behind the killings. Hurst's statement will take the tribunal into the area of CFs (contact forms) MISRs (Military Intelligence Source Reports) and RIRACs (Special Branch SB50 forms provided to the Army for intelligence purposes) - all of which remain secret.
How many classified registers of MISRs the tribunal will be allowed to see in London remains unclear. It is an area Judge Smithwick and his team of lawyers might have chosen not to enter, but circumstances have propelled them into the bowels of the Ministry of Defence.
Whether they turn up documents linking Belfast republican Freddie Scappaticci - known as 'Stakeknife' to his handlers - to one or more Garda officers then serving along the border is the key question.
Hurst is not allowed to speak about his military service in either Londonderry or Enniskillen between 1982 and his last tour of duty in 1990 as a member of the Force Research Unit. What we do know is that one Garda officer in particular has denied any link with Scappaticci and the IRA.
But the tribunal has other evidence which it can more easily produce which corroborates the reports that both Harry Breen and Bob Buchanan were concerned about the activities of the same Garda - to the extent that Buchanan raised his concerns with an officer of equivalent rank in Louth six months before his murder.
Last week, following just one day of evidence-taking in public, the tribunal's lawyers travelled to London to interview at least one Army officer who, like Hurst, served here in the 1980s as a FRU officer and who reportedly was Scappaticci's handler.
Whether Major David Moyles freely volunteered that information to Smithwick's lawyers, or advised them that Scappaticci admitted to him that he met the Garda officer under suspicion, we will not know for some time.
Ten days ago, after former RUC assistant chief constable David Cushley had given evidence challenging the evidence of another former assistant chief constable, Archie Hayes, about arrangements for the fateful Dundalk meeting, a lawyer representing Garda commissioner Martin Callinan questioned him, not about arrangements for the meeting, but about Scappaticci. Did Mr Cushley know Scappaticci? Had he had dealings with him?
Is there some matter about Scappaticci which could emerge that would cause acute embarrassment to the Garda commissioner?
Ian Hurst may know the answer, but for the moment his testimony is denied to the tribunal following several High Court injunctions and other legal mechanisms which restrain his availability to give evidence. Witnesses are expected to appear in Dublin over four days this week as the meaty work of the tribunal gathers pace.