The three brothers were in their south Armagh home watching TV when the loyalist gunmen burst in. John Reavey (24) didn't stand a chance. He died in a hail of bullets in the chair where he'd been sitting. Brian Reavey (22) ran into another room but fell into the fireplace after being shot. Anthony Reavey (17) managed to reach a bedroom and hide under the bed. They shot him several times, then left, believing he too was dead. He survived but died three weeks later in hospital.

Tuesday marks the 35th anniversary of the triple murder, but the family still craves justice. Eugene Reavey hopes the truth will finally be established this year when a Historical Enquiries Team (HET) report uncovers the disturbing nature of collusion between his brothers' killers and the British state.

The attack was carried out by the UVF's infamous Glenanne gang, which operated in a murder triangle between south Armagh and mid-Ulster. Made up of security force members, it was run by British military intelligence. It was responsible for up to 120 killings, including the Dublin and Monaghan bombings.

"In south Armagh, there was no UVF," Eugene Reavey says. "There were RUC and UDR men who wore one uniform by day and another at night." The shooting on 4 January 1976 was the beginning of a horrific 24 hours in the north in which 16 people were murdered.

Ten minutes after the attack on the Reaveys, other members of the Glenanne gang burst into the O'Dowds' home, 15 miles up the road in Gilford, Co Down. The family were gathered around the piano for a post-New Year sing-song. Four children were in the room.

The gunmen killed Barry O'Dowd (24), his brother Declan (19) and uncle Joe (61). The next evening, stating retaliation for those attacks, the IRA stopped a minibus carrying factory workers at Kingsmill in south Armagh, ordered the men out, and opened fire. Ten innocent Protestants were killed (see panel on opposite page).

Sadie Reavey (88) says her entire family could have been wiped out by the Glenanne gang. "Normally, on a Sunday evening, my eight sons and four daughters would all be at home but that night I took everybody except the three boys to visit my sister."

The family's torment was only beginning. "When we were coming back from the morgue, the RUC and British army stopped us. They took my brothers' blood-soaked clothing from the car and danced on it on the road. They stuck their guns into our backs. We suffered constant security force harassment," Eugene Reavey says.

His father made the surviving five brothers swear not to retaliate nor join any republican paramilitary group: "A few days after the shooting, a Protestant publican in Markethill phoned my daddy with the names of the killers. He kept those names to himself until he was on his deathbed."

Sadie Reavey misses her sons as much as ever. "I've run after enough young fellows with red hair on the street thinking they're Anthony. I still look in the shops for a jersey that would suit John or a shirt for Brian. For years, I still set the table at night for those boys."

Eugene Reavey says: "Every Sunday at the Gaelic football, I see a player who reminds me of Brian – the way somebody catches a ball or shows a burst of speed."

On Reavey's way to the morgue to collect his brothers' bodies, he stumbled into the Kingsmill massacre when the bodies were still on the road. This led to rumours that he was involved in the attack.

In 1999, the Rev Ian Paisley used parliamentary privilege to accuse him of organising the atrocity. Paisley said his information came from an RUC document. The police said there was no such document and Reavey was entirely innocent. The only survivor of Kingsmill, Alan Black, says he knows Reavey wasn't involved.

"Ian Paisley was given a false document but it's time for him to retract his accusation," Reavey says. "Paisley has made admirable compromises in recent years, making peace with old enemies, entering government with Sinn Féin. He's regarded as a statesman. He must do the decent thing."

Over the years, allegations circulated that the dead brothers were paramilitaries. "No member of our family has ever been involved with the IRA," Reavey says. "Sometimes, I wish my brothers had been because then they would have been out taking their chances and their deaths would have been easier to live with."

The HET was the first state representative to show any interest in the triple murder. "In three decades until then, nobody crossed our door," Reavey says. "The police file on the killings was only a page-and-a-half long."

The HET report, due out last August, was delayed because investigators found collusion between the Glenanne gang and the security forces was far more extensive than previously believed. The report will unequivocally state the Reaveys had no IRA connections. The HET has already apologised to the family for the security forces' "appalling behaviour".

Three gunmen with four weapons entered the Reaveys' home that night. The guns had been used in dozens of other attacks, including the murder of three Catholics a fortnight earlier in Donnelly's bar in Silverbridge, south Armagh.

The three gunmen who entered the Reaveys' house were Robert McConnell, a UDR member later killed by the IRA; RUC officer James Mitchell who died two years ago; and another RUC man who is still alive and can't be named for legal reasons. The getaway car was driven by Mitchell's young female lover.

Other members of the Glenanne gang included ex-UDR man Robin 'the Jackal' Jackson, who travelled around the country in a chicken lorry, and RUC man Billy McCaughey. Both have since died of lung cancer but many other members of the gang are still alive.