Gareth O'Connor, 24, disappeared in May 2003. His body was recovered from Newry Canal two years later.
The father of two disappeared on his way to Dundalk Garda station where he was due to report as part of bail conditions set after he was charged with Real IRA membership.
At the time, detectives believed he had been killed by the Provisional IRA.
Earlier this year, a hearing was told that 10 people named in police documents relating to the murder have had their identities protected.
Chief coroner John Leckey asked the Police Service of Northern Ireland's legal team to find out why the names had been redacted,
Some of the people whose names have been withheld are witnesses and some may have been suspects in Mr O'Connor's death.
On Monday morning, Mark Robinson, counsel for the PSNI, told Mr Leckey that more documents have now been found at Gough Barracks in Armagh and more time would be needed to assess them.
The hearing was told the PSNI had plans to write to the coroner about this over the next 14 days. However,  when asked why it would take 14 days to send the letter, Mr Robinson said it could be sent within the next seven days.
Paul Dougan, solicitor for Mr O'Connor's family, said there was "absolutely no adherence to timescale in this case". He added that delays in the pre-inquest hearings had become "depressingly predictable".
Coroner John Leckey also questioned how the latest file had turned up now and asked "why hasn't the PSNI an inventory of all material available in Northern Ireland, making it easier to access?"
A pre-inquest hearing into the murder of a County Armagh man 10 years ago has been adjourned again.