An offer by MI5 to disclose files relating to the murder of the Belfast solicitor Pat Finucane could be significant, an SDLP MLA has said.
Alex Attwood was speaking as details emerged of US cables published on the Wikileaks site.
They shed new light on the inquiry into the murder of the Belfast solicitor by loyalist paramilitaries in 1989.
There is still disagreement between the government and the Finucane family about an inquiry.
The murder of Mr Finucane, 39, by the Ulster Freedom Fighters remains one of the most controversial of 30 years of violence.
Loyalist paramilitaries shot him 14 times as he sat eating a Sunday meal at home, wounding his wife in the process. The couple's three children witnessed the 1989 attack.
The killing was controversial because of the allegations of collusion between loyalist paramilitaries and members of the security forces.
The leaked diplomatic cable said the former head of MI5 told US envoy Mitchell Reiss that her agency would turn over all the evidence it had on Mr Finucane's killing, provided any inquiry into the killing was conducted under the terms of a law passed in 2005, the Inquiries Act.
However, Mr Attwood said while the MI5 files offer could be significant, it might not lead to a full disclosure of the facts.
"Files have a habit of being cleaned up," he said.
"In any case an inquiry under the model proposed by the British government, which is not the model proposed by the Cory investigation into this matter, will not disclose in full what many people including myself believe needs to be disclosed in order to get to the truth."
Amnesty International said the leak strengthened the case for a full, independent public inquiry.
Its UK director Kate Allen said the government's failure to hold an inquiry was "outrageous".
She added: "These new revelations show concerns that elements within the UK security establishment have sought to avoid an inquiry and, further, that MI5 holds important files on the killing which have yet to be disclosed."
'Rock solid evidence'
The leaked US diplomatic cables also reveal that the Irish government believed Gerry Adams and Martin McGuinness had advance knowledge of the Northern Bank robbery.
The messages, which were obtained by the Wikileaks website, date from 2005 and were published in the Guardian.
They reveal that the Irish government "had rock solid evidence" Sinn Fein leaders Mr Adams and Mr McGuinness were members of the "IRA military command".
Mr Adams said he had repudiated the allegation before and it was not true.
£26.5m was stolen from the Northern Bank in Belfast in December 2004.
In a cable on 4 February 2005, two months after the robbery, the US ambassador to Dublin, James Kenny, reported that a senior Irish government official told the embassy of Taoiseach Bertie Ahern's concerns about Mr Adams and Mr McGuinness.
The cable claimed the official in the department of justice told the ambassador "that the GOI (government of Ireland) does have 'rock solid evidence' that Gerry Adams and Martin McGuinness are members of the IRA military command and for that reason, the taoiseach is certain they would have known in advance of the robbery".
Mr Adams has always denied being a member of the IRA, although Mr McGuinness has said he was second in command of the organisation in Londonderry in the early 1970s.
On Monday he said: "The taoiseach at the time, in a quite disgraceful outburst, made that assertion (about the Northern bank robbery).
"I repudiated it then, as did Martin, we both denied it very, very strongly - it isn't true.
"So I saw this and still see this as part of Fianna Fail's attack or fight back against Sinn Fein at that time."
Also contained in the leaks was the belief of Irish police that the IRA used the Republic's Celtic Tiger economic boom to diversify into "more sophisticated business enterprises" by buying up properties in London, Dublin and Spanish resorts.
Responding to news of the leaked cables, Jim Allister of the TUV said the Sinn Fein leaders should not be in government.
"It is clear no serious attempt was ever made to solve the Northern Bank robbery at a level which carried any political risk," he said.
"On a day when we hear hype over marvellous PSNI/Garda cooperation, the obvious question is what actual cooperation was sought and obtained in bringing to justice those implicated in this terrorist robbery?"