Documents have not been disclosed in time for Monday's preliminary inquest into the death of Real IRA man Kieran Doherty, a lawyer said.
The 31-year-old had been stripped and bound before he was shot dead.
His body was found dumped in Braehead Road in Londonderry on February 24, 2010.
The Real IRA claimed responsibility for the attack.
A preliminary hearing of an inquest into the death was held in Belfast.
A lawyer for the coroner said: "It was hoped that disclosure would be able to be made in advance of today.
"That has not proved possible."
Peter Coll, a barrister for the security forces, said work was ongoing.
Coroner Brian Sherrard said: "I understand the obvious frustration that the family can feel.
"This is a matter that the court is taking a very active interest in at the minute."
In the weeks before his death, Mr Doherty claimed MI5 attempted to recruit him as an agent and members of his family have raised concerns the UK security services could have played a role in his death.