The family of murdered Catholic teenager Eileen Duffy are to sue the Ministry of Defence, the Police Service of Northern Ireland and a Protestant church deacon in an action alleging "unlawful killing".

The family of another couple slaughtered by the same Ulster Volunteer Force gang a year later are also taking the legal action. Both families have been granted legal aid to pursue the case.

In the legal papers, seen by the Belfast Telegraph, the families claim the church deacon, Alan Oliver, 46, was a British intelligence agent. In previous court papers Oliver has denied being an informer.

The two families plan to issue writs for "unlawful killing, negligence, breach of statutory duty and misfeasance in public affairs".

Oliver, a committed Christian and evangelical, is a deacon in Portadown Elim Church.

Eileen Duffy was shot dead along with Katrina Rennie, 16, and Brian Frizzell, 29, by a masked gunman in a military-style jacket who had pulled up in a van outside a mobile shop in 1991. The brutal shootings shocked the world and were one of a series of violent incidents during the troubles in Craigavon, County Armagh, Northern Ireland.

Tess and Charlie Fox were shot dead by the same UVF gang as they prepared for bed at their home in Moy, County Tyrone, the following year. Their son Patrick had just been jailed for possessing an IRA bomb.

Another of their sons, Anthony Fox, told the Belfast Telegraph: "We are delighted that Alan Oliver is at last going to see the inside of a courtroom.

"We believe that our parents were slaughtered by a sectarian serial killer, and we are also finally getting the chance to hold the state to account for their shameful activities."

Eileen Duffy's brother Brendan said: "We hope that, after all these years, we can secure justice for Eileen. She was only 19 when they took away her life. She was buried in the dress she had bought to go to dances. We have no doubt that the gunman was a British agent."

Both shootings were carried out by the UVF's Mid-Ulster Brigade.

Brendan Duffy said: "I found Eileen slumped on the floor of the shop, shot in the head. Her face was badly swollen and blood was pumping out of her head and ears. I tried to resuscitate her but in my heart, I knew she was gone.

"The police took almost half an hour to get to the scene when it was a five-minute walk from the police station."

Brendan has accused the police of failing to investigate the murder adequately and said the only contact the family had with officers after the massacre was when they were asked if they wanted their daughter's clothes back.

Oliver has been previously named in court papers as the gunman.

Brendan Duffy said: "Alan Oliver claims to now be a Christian. If that's so, he needs to come clean and tell us who his handlers were."

Anthony Fox said: "We are calling on Elim Church to get rid of Alan Oliver. If he has reformed, then he should tell the truth about what he's done."