The only survivor of the Kingsmills massacre and a sister of one of those killed have said they believe efforts to investigate the IRA atrocity have been hindered.
They blame political interference and a desire to protect agents.
Ten Protestant workmen were killed when their minibus was ambushed in south Armagh in January 1976 .
The latest sitting of the inquest into the deaths had heard of further delays.
Alan Black, who survived the massacre and Karen Armstrong whose brother, John McConville was killed, spoke after Thursday's hearing.
"I have suspected all along there were agents involved and the fact that two people got On-The-Run letters, reinforces that," said Mr Black.
"This is a dirty game player by dirty people but we must go though the proper processes."
"This case has never been handled correctly from the start," he said.
"People have been protected because of Kingsmills and their involvement. I believe agents are being protected " added Mrs Armstrong.
New lines of inquiry into the massacre are being pursued by police after victims intervened, a lawyer told the inquest on Thursday.
Families made representations to detectives and prosecutors investigating the attack.
Background to Kingsmills
No-one has been convicted of the attack.
Those on board were asked their religion and the only Catholic was ordered to run away.
An inquest began 40 years after the killings but was suspended following a major forensic breakthrough by police.
Police believed a palm print found on a vehicle used during the killing belonged to a suspect arrested earlier this year, a previous sitting of the inquest was told.
Legal counsel to the coroner reviewing the killings told Thursday's session in Belfast that the coroner has written to the PPS asking for an indicative time scale when inquiries may be completed and when a decision can be expected.
The inquest has outlined suspected linkages between weapons used at Kingsmill and other shootings.
Lawyers for the families of those killed have appealed for top secret intelligence material to be released to the inquest.