MI5 is to be legally represented at the public inquiry into the murder of solicitor Rosemary Nelson.
The solicitor died after a booby-trap bomb attack on her car
She died after loyalists planted a booby-trap bomb underneath her car outside her Lurgan home in March 1999.
Retired judge Sir Michael Morland is chairing an inquiry into allegations of security force collusion in the murder.
MI5 has argued that it would have assumed a lead role in Northern Ireland by the time of the inquiry next year and should be represented.
Mrs Nelson's brother, Eunan McGee, said the decision had taken the family by surprise.
Their concern is that MI5 might consequently be able to apply to remove sensitive or classified information in the papers.
WHO WAS ROSEMARY NELSON?
A Catholic solicitor who came to prominence representing high profile cases
These included working for the nationalist Garvaghy Road Residents' Coalition in the dispute with Orangemen over Drumcree
The 40-year-old mother-of-three was killed in a booby-trap car bomb near her home on 15 March 1999
A splinter loyalist group, the Red Hand Defenders, said it carried out the murder
"It baffled us to tell you the truth because we have a few concerns," he said.
"The fact that they will see all the documentation and can apply for a redaction in documentation is a major concern. After all this has been about one issue and one issue only. We need to get to the truth."
The inquiry into the murder is to begin its full hearings in January 2007, nearly a year later than originally planned.
There have been allegations of security force collusion in the killing of the 40-year-old solicitor because of her role as the legal representative for the nationalist Garvaghy Road Residents' Coalition and other high profile cases.
Sir Michael and his colleagues - ex-chief constable of South Wales Sir Anthony Burden and Dame Valerie Strachan, former chair of the board of Customs and Excise - will examine claims that police ignored death threats against Mrs Nelson.
Her murder was carried out by the Red Hand Defenders, which is a cover name for the Ulster Defence Association and Loyalist Volunteer Force.
The government agreed to set up an inquiry into Mrs Nelson's death following the recommendations of retired Canadian Judge Peter Cory.
SDLP Upper Bann assembly member Dolores Kelly said on Thursday that news of MI5 involvement was "profoundly worrying".
She asked whether the security service had intelligence about the murder.
"Or is this just a warning to us all about the enlarged role that MI5 wants to have in the north in the future - involving themselves in more and more aspects of policing?" she said.