The Police Service of Northern Ireland (PSNI) has been accused of handing over impenetrable, anonymous and unverified intelligence to an inquiry into Garda collusion with the IRA in the Irish Republic.
Lawyers for the Garda Commissioner Martin Callinan launched a scathing attack on the force, accusing it of failing the families of the two most senior RUC officers killed in the Troubles.
As the long-running Smithwick tribunal sat for its last day, senior counsel Diarmaid McGuinness criticised PSNI Assistant Chief Constable Drew Harris for claiming intelligence revealed there were several unnamed gardai colluding with the Provos.
"These strands of evidence are worthless and weightless so as to provide proof of any matter," Mr McGuinness said in his closing submissions.
"They cast the gravest shadow over bona fides, the willingness and the ability of the PSNI to co-operate with the tribunal."
The tribunal has been probing any links between gardai or Garda civilians and the Provos' ambush of Chief Superintendent Harry Breen and Superintendent Bob Buchanan on March 20, 1989, shortly after a meeting with a senior garda in Dundalk.
Three named former Garda officers - Owen Corrigan, Leo Colton and Finbarr Hickey - strongly deny any allegations of collusion.
Mr McGuinness maintained that Mr Harris told the tribunal there was no RUC intelligence at the time of the murders to suggest collusion.
Twenty-four years after their murders, and eight years in to the inquiry, the chairman was presented with what the lawyer called "an impenetrable, anonymous, unverified bunch of intelligence".
Mr McGuinness said the PSNI has refused to share the evidence in full but asked the tribunal to make a judgment on it. He said: "It beggars belief as to how you are expected to come to adjudication, not merely in relation to this evidence, but in relation to the issue of collusion as a whole, having regard of the actions of the PSNI."