BBCAn investigation into the murder of 10 men in County Armagh failed to arrest and interview 11 men identified by intelligence, the police ombudsman has found.
Last year an inquest found the atrocity, in Kingsmills, was an overtly sectarian attack mounted by the IRA.
Police Ombudsman Marie Anderson said the failures took place against a backdrop of “wholly insufficient” resources.
The textile factory workers were shot dead when a gang ambushed their minibus near Kingsmills in 1976.
Their families, and some unionist politicians, have called for an public inquiry.
What happened at Kingsmills?
The attack took place on 5 January 1976, just after 17:30 GMT.
A red Ford Transit bus was carrying the men home from their workplace in Glenanne, along the rural road to Bessbrook.
As the bus cleared the rise of a hill, it was stopped by a man standing in the road flashing a torch.
As the vehicle came to a halt, 11 other men, all masked and armed, emerged from hedges around the road.
The IRA men ordered the passengers out of the bus demanding to know the religion of each of the men.
One of the workers, who identified himself as a Catholic, was told to leave.
The gang then opened fire on the remaining passengers, killing 10 Protestant workmen and seriously wounding another.
No-one has ever been held to account for the murders.
Who were the Kingsmills victims?
The 10 men who were killed at Kingsmills were:
- John Bryans
- Robert Chambers
- Walter Chapman
- Robert Freeburn
- Reginald Chapman
- Joseph Lemmon
- John McConville
- James McWhirter
- Robert Walker
- Kenneth Worton
A memorial service is held in south Armagh every year to remember them.
Only one man, Alan Black, survived the shooting.
He was shot 18 times and spent months in hospital recovering from his injuries.
Who carried out the Kingsmills murders?
In 2011, a report from the Historical Enquiries Team in Northern Ireland said the IRA was responsible for the attack.
It concluded that it had been a purely sectarian attack.
An inquest last year found there was no evidence of collusion or state involvement and that the attack was carried out by a unit consisting of at least 12 members of the IRA, pretending to be an army patrol.
Shortly after the attack the so-called South Armagh Republican Action Force claimed responsibility for it. The coroner, Judge Sherrard, said that was a lie.
The IRA has never admitted involvement and was supposed to be on ceasefire at the time of the attack.
The judge at the inquest added Kingsmills was “ostensibly in direct response” to attacks on the Catholic Reavey and O’Dowd families by loyalist terrorists one day before, though Kingsmills was not spontaneous and had been planned “well in advance”.
Throughout the Troubles, loyalist and republican paramilitaries carried out tit-for-tat murders, killing people simply based on their religion.