Sir Desmond de Silva promised his inquiry into the murder of Belfast solicitor Pat Finucane in 1989 would be hard-hitting.

It is true, as David Cameron admitted yesterday, that it reveals deeply shocking levels of collusion by the state. That much, however, was already known as a result of previous investigations and admitted by the current UK Government. The new information is the extent to which the British Army, security services and RUC special branch colluded with paramilitary organisations.

That goes beyond shocking. It is difficult to see the failure to warn known assassination targets, including Mr Finucane, that their lives were in danger, as anything other than chillingly inhumane, chilling even by the standards of sectarian violence in Northern Ireland at the time.

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The key question now is whether this review has finally uncovered the whole truth. Mr Cameron claimed the report had been an exercise in opening up the Government, the security services and the police to the maximum extent possible. For the Finucane family, despite Sir Desmond having access to previously undisclosed documents, there is no means of verifying conflicting accounts without public cross-examination of witnesses.

Mr Finucane's widow Geraldine believes Sir Desmond has given the benefit of the doubt to the security services. His finding that the high-level ignorance among those on the higher echelons of authority was possibly intentional and that Government ministers were misled leaves unanswered the vital question of just how far up the chain of command the collusion reached.

That degree of transparency can only be achieved via a public inquiry. It is true that some lessons have been learned from this appalling case and other murders in Northern Ireland. Most obviously, there is now a statutory framework for the operation of agents.

The primary purpose of a public inquiry is to establish the facts but it is by doing so under public scrutiny that confidence in the rule of law and the workings of the state is restored. That is particularly so in the most contentious cases where unacceptable practices have been carried out and then covered up by public bodies, particularly the police and the armed forces. We have seen the cleansing effect of uncovering and acknowledging disgraceful failings in both the Bloody Sunday and Hillsborough inquiries.

In dismissing the case for a public inquiry yesterday, the Prime Minister cited expense as a factor. It would be an undoubtedly lengthy operation costing many millions of pounds but that must be weighed against the cost of failing to be fully transparent.

The de Silva report is a poor substitute for the inquiry planned by the Labour Government of Tony Blair in 2005 as part of an agreement between the UK and Irish governments.

Its limitations were summed up by Geraldine Finucane when, in accepting Mr Cameron's apology, she said she still did not know exactly what he was apologising for.