TWO senior members of the IRA were responsible for the Kingsmill massacre, an inquest has been told.

Brian Keenan and Seamus Twomey were named in court by one of their former comrades as the men behind the atrocity.

Giving evidence to Belfast Coroner's Court from a secret location via Skype, ex-IRA man and police informer Sean O'Callaghan said many within the IRA at the time – including Sinn Féin leader Gerry Adams and former Stormont minister Martin McGuinness –were critical of the attack.

"Adams and McGuinness were very critical.

"Adams referred to Kingsmill as another one of Brian Keenan's mess ups," Mr O'Callaghan told the court.

His evidence, during the second week of an inquest into the 1976 murders of 10 protestant workmen, provided an insight into the workings of the republican terror group at that time.

The men were shot dead by an IRA gang after their minibus was stopped near the County Armagh village of Kingsmill.

Those who were killed were travelling home from work in a textile factory when they were ambushed.

An eleventh man, Alan Black, survived despite being seriously injured.

Mr O'Callaghan said the IRA was a "very fractured" organisation at that time with many members on the run and others in prison.

He told the court Keenan was IRA quarter master and was running the IRA bombing campaign.

He was also the link man with Libyan dictator Muammar Gaddafi, the court heard.

Mr O'Callaghan described Keenan as "deeply sectarian and ruthless" and said he was responsible for the massacre.

He also said Keenan disagreed with "the old leadership of McGuinness and Adams."

Twomey was described by Mr O'Callaghan as IRA chief of staff.

He said he had been told that Twomey had authorised the Kingsmill attack.

Both Twomey and Keenan are now dead.

Mr O'Callaghan told the court that the attack had not been sanctioned by the IRA army council, but added that it did not have to be as there was a policy in place that permitted retribution for sectarian attacks.

"It is my belief the IRA army council did not sanction [Kingsmill] but at that time they wouldn't have had to," he said.

He added that if a volunteer had taken weapons to carry out an unauthorised attack there would have been "severe repercussions".

When asked by a lawyer for the victims' families did he mean "men would have been found dead", he replied: "Absolutely."

Mr O'Callaghan said he did not have any direct knowledge about the massacre as he had left the IRA the previous year, rejoining in 1979.

However he said he gleaned the information from senior republicans, including Martin Ferris.

When asked by a lawyer for the victims' families if the IRA was ever likely to apologise, Mr O'Callaghan said: "Only if the leadership believe it is politically expedient to do so."

Earlier on Friday the officer in charge of the original investigation, Detective Chief Inspector James Mitchell, apologised to the victims' families for "weaknesses" in the original police probe.

When asked by a lawyer if he had anything he wished to say to them, Mr Mitchell said: "Given some of the weaknesses identified - most of that was down to the workload we had at the time, and the very depleted manpower - I apologise if they didn't get the satisfaction they hoped they would have got."

It emerged during his evidence that no direct evidential link exists between the suspected getaway van and the murder scene.

Mr Mitchell said there was no record to show that police had received the results of any forensic tests carried out on soil, broken mirror and paint samples gathered from the scene and the van.