Alan Black, the only survivor of Kingsmill, believes that IRA members involved in the massacre were double agents working for the British state. He believes there was a "cover up" and that British security forces knew the massacre was going to happen but allowed it to. Karen Armstrong, sister of victim John McConville, said: "A lot of people were being protected back then and they still are".It has been suggested that the gunman with the English accent could have been British Intelligence officer Robert Nairac.
John Weir, a former RUC officer and member of the "Glenanne gang", claims he discovered that British Intelligence, through Nairac, was "playing republican and loyalist paramilitaries off against each other to draw the IRA away from it's war against the state and it's main political goals".
In an August 1975 letter to Tony Stoughton, chief of the British Army Information Service in Northern Ireland, Wallace writes:
There is good evidence the Dublin bombings in May last year were a reprisal for the Irish government's role in bringing about the [power sharing] Executive. According to one of Craig's people [Craig Smellie, the top MI6 officer in Northern Ireland], some of those involved – the Youngs, the Jacksons, Mulholland, Hanna, Kerr and McConnell – were working closely with [Special Branch] and [Military Intelligence] at that time. Craig's people believe the sectarian assassinations were designed to destroy Rees's attempts to negotiate a ceasefire, and the targets were identified for both sides by [Intelligence/Special Branch]. They also believe some very senior RUC officers were involved with this group. In short, it would appear that loyalist paramilitaries and [Intelligence/Special Branch] members have formed some sort of pseudo gangs in an attempt to fight a war of attrition against the IRA by getting paramilitaries on both sides to kill each other.
In a further letter of September 1975, Wallace wrote that MI5 was backing a group of UVF hardliners who opposed the UVF's move toward politics. He added:
I believe much of the violence generated during the latter part of last year was caused by some of the new [Intelligence] people deliberately stirring up the conflict. As you know, we have never been allowed to target the breakaway UVF, nor the UFF, during the past year. Yet they have killed more people than the IRA!