The Attorney General has backed a plea bargain deal that frees Northern Ireland's first murder case supergrass in decades, released after admitting his role in a pair of horrific murders.

Baroness Scotland says she is satisfied with an arrangement that saw Mark Burcombe sentenced to 30 months in jail — time he had already served on remand — in return for giving evidence against another man accused of the murders.

The families of Andrew Robb and David McIlwaine, the teenagers killed in a horrific knife attack in Tandragee, Co Armagh, eight years ago, were outraged at the sentence.

Sinn Fein president Gerry Adams had asked the Attorney General to review the case and consider an appeal to give Burcombe a stiffer sentence.

But she told the MP that she is satisfied the Prosecution Service has "acted correctly" and promised to explain her reasons when the case is finally closed, probably later year. She also backed the sentence imposed on Burcombe for his guilty plea for conspiring to assault Andrew Robb — although she noted that will be reconsidered if Burcombe does not testify.

"I have every sympathy for the families of David McIlwaine and Andrew Robb," she wrote in a letter to the West Belfast MP.

"I can only begin to imagine the impact these dreadful offences have had on them.

"But, like the judge, I can only assess the correctness of the sentence imposed on Burcombe having regard to what he admitted to doing, not what the families believe he may have done."

David McIlwaine's father, Paul, said he understood Baroness Scotland's reasons, but he continues to believe Burcombe's statement to police has left out important information — including the role of a UVF leader suspected of being an informer.

"I understand where Baroness Scotland is coming from. She can only work on the information she has," he said.

"Depending on what happens in the trial, she'll look at it further. I believe she can read between the lines, but she can only go on what's being said.

"But this is a disgusting situation. It's a disgrace."

Burcombe (27), from Ballynahinch Road, Lisburn, is scheduled to give evidence against Steven Brown, also known as Steven Revels, in the autumn.

Brown, also 27, from Castle Place, Castlecaulfield, is accused of murdering both teenagers.

Mr McIlwaine assumes Burcombe has been freed because he has spent more than 30 months in prison awaiting trial, although he has not been notified by the authorities.

While sentencing Burcombe in June, Mr Justice Hart referred to the contents of Burcombe's latest statement to police.

He told police that Brown and another man who later committed suicide, Noel Dillon, killed Andrew Robb because he made a disparaging remark about Richard Jameson, the UVF leader in Portadown who had been murdered a month earlier.

He said David McIlwaine was murdered because he witnessed the attack.

Burcombe claimed that he thought Andrew Robb was going to get a beating for the remark, and said that he did not know a murder would take place.