IT was a spur-of-the-moment decision to stop in the old cobbled square which looked so beautiful in the moonlight that summer's night. The four Australian tourists just couldn't resist parking their car and exploring the historic centre of Roermond.

That decision cost two of the travellers their lives. As they walked around the Dutch town and chose a restaurant for dinner, they didn't know the IRA was watching. Ninety minutes later, Stephan Melrose (24) and Nick Spanos (28) lay dead, their blood splattered over the old square they had so admired.

Melrose's wife Lyndal and Spanos's girlfriend, Vicki Coss, watched helplessly as two IRA gunmen, clad in black and wearing balaclavas, opened fire on the men. The Australians knew nothing of the IRA or that British soldiers based in Germany regularly crossed the border to socialise in Roermond.

Melrose and Spanos were tall, well-built and clean-shaven with short hair. Their appearance and their car's English-registration plates led the IRA to believe they were soldiers. Had the Australians the chance to utter a word as the gunmen approached, their accents would have saved them. But the IRA team's recklessness denied them even that.

Twenty years later, the Melrose family have travelled to Roermond and to Ireland, in search of answers to questions that have haunted them since the 1990 shooting. Justice has proved elusive. Four Irish people were charged with the double murder. Three were acquitted and the conviction of the fourth was over-turned on appeal.

There are strong suspicions British intelligence had advance knowledge of the attack and failed to act in order to protect agents in the IRA operating in Europe. The family have also long wanted to meet Sinn Féin leaders to discuss the attack.

Melrose and Spanos were Australian lawyers working in London. They'd been on a weekend break to Amsterdam, visiting museums and viewing Van Goghs. They were driving to Calais to catch the ferry back to England when they stopped at Roermond.

They had dinner in the Tin San Chinese restaurant, ordering a huge feast and drinking wine and beer. "They were laughing and joking – they were so happy," recalled waiter Shi Keung-Ko. Afterwards, they strolled back to their car. The others climbed inside but Melrose wanted to photograph the square's magnificent 18th century town hall.

He was setting up his camera and tripod when two gunmen, armed with an AK47 and a Ruger, ran across the square firing. Most of Melrose's head was blown away. One gunman then walked to the car side door and shot Spanos. The assailants made off in a stolen Mazda, driven by a third IRA member.

Lyndal Melrose, her dress covered in blood, ran towards café owner Henri Tijsje-Kloisen screaming: "I've been married nine months and my husband's dying." Three weeks later, Donna Maguire (24) and Paul Hughes (26) from Newry, Seán Hick (30) from Glenageary, Co Dublin, and Gerard Harte (26) from Lurgan were charged with the double murder. The prosecution claimed that the four planned the killings, arranged forged passports and driving licenses, a getaway car and a safe house.

The court heard that Maguire, then the most wanted female terrorist in Europe, was a key figure in the operation. Eyewitnesses also provided testimony. Despite what the prosecution believed was a strong case, only Harte was convicted and his conviction was overturned on appeal.

"The family's journey to Holland and Ireland was an attempt to find explanations," says award-winning investigative journalist Ross Coulthart, who accompanied them. The trip was filmed as a documentary for Australia's Channel Seven and will be screened next Sunday. It includes a confrontation between Coulthart and Gerry Adams.

Moving welcome

After the shooting, the IRA apologised. At the time of the murder trial in Holland, the Melroses were too emotionally distressed to attend and also lacked the resources for the trip. But a fortnight ago, Stephan Melrose's parents – Roy and Beverley – his sisters Helen and Susy, and Susy's husband Ian visited the murder spot in Roermond.

"They were given a moving welcome by people in the town. There wasn't a dry eye in the square when they laid a wreath at the spot where Stephan was killed," says Coulthart.

The family also had "a very candid meeting" with Jo Lauman, the Dutch prosecutor in the case. "He insisted that a very strong case had been presented against the accused," says Coulthart. The film-makers also tracked down eyewitnesses and interviewed former British agents in the IRA.

"Just after the shooting in 1990, an Australian source with good links to British intelligence suggested to me that the British had details of the IRA cell operating in Europe, that they had information on its key members, and that they could possibly have stopped the attack. This is supported by ex-British agents we have interviewed for the programme," says Coulthart.

There is speculation that at least one member, and probably several, of the IRA team in Europe was working for British intelligence. Coulthart says he has also information from former British intelligence figures that at the most senior leadership level in the IRA, one in two leaders were British agents.

"There is a strong possibility that British intelligence had advance warning of this attack but didn't stop it," says Coulthart. "The question must be asked whether two Australians were sacrificed to protect informants in the IRA. If this is so then British intelligence is every bit as responsible and every bit as evil as the gunmen who pulled the trigger. Our programme is very critical of the IRA but it must also be said that the British fought a very dirty war."

Gerry Adams and Martin McGuinness were on the IRA Army Council at the time of the Australians' deaths. "The Melroses wanted to meet both men," says Coulthart. "We wrote to them asking about this and also to interview them on camera. They refused, saying it would be like a soap opera.

"We then wrote requesting that Gerry Adams meet the family without cameras present. We received no reply. When we came to Ireland we repeatedly asked Sinn Féin for meetings but they never got back to us."

However, the Australian film crew door-stepped Gerry Adams as he launched a celebrity Poc Fada at Stormont. Adams said he regretted the killings and expressed sympathy to the families. But he insisted he was never in the IRA and his role in past events was "an open book".

Coulthart says: "I asked him why Brendan Hughes, one of his closest friends, had said he was an IRA leader and had ordered murders. He replied, 'Brendan's dead now'. I asked him why so many of his former IRA colleagues were saying he'd been an IRA leader and he said, 'You'll have to ask them that.'"

When Coulthart asked Adams if he was on the Army Council in 1990, he replied: "You shouldn't be coming here and presuming to make assertions and allegations like that."

The film's producer, Mick O'Donnell, was stunned at Adams' tone: "He thought it impudent to be even asked the question when it was entirely reasonable. It seems he doesn't get a rough ride from some Irish media, that there is a desire not to pick at the scabs.

"It's very different in Australia. When there are suggestions that any of our politicians are involved in wrong-doing of a far less serious nature than Gerry Adams is accused of, it's open season."

Coulthart showed Adams a secret document released by the British Public Records Office under the 30-year rule. It disclosed that in 1972 Adams as "an IRA representative" met MI6 agent Frank Steele for ceasefire discussions at a house on the Derry-Donegal border. Adams told Coulthart he had met British government, but never British intelligence representatives.

While Adams refused to meet the Melroses, a former Belfast IRA member and bomb-maker did. Tommy Gorman was one of the 'Magnificent Seven' who escaped from the prison ship the Maidstone. Gorman, who now lives in Donegal, told the Sunday Tribune: "I thought it important to meet this family.

"Their son was cut down with a wonderful life ahead of him, as part of a conflict that was nothing to do with him. Had it been my son, I'd have found it very hard to handle. The family shouldn't have been left hanging in mid-air, they deserved some sort of explanation.

"I told the Melroses that the volunteers responsible didn't set out to kill Australian tourists, that it made no sense from any angle – moral or propaganda – that it was a huge mistake. I wasn't part of the IRA leadership, I wasn't part of any European unit. But, as an IRA volunteer, I shared part of the collective responsibility for what happened. You can't distance yourself just because you don't like one particular operation.

Healing process

"In 1990, people like myself were lobbying for a ceasefire because we believed the armed struggle was going nowhere. What I find disgraceful is that the leadership was sending volunteers to Europe, to kill and be killed, when they were involved in secret talks with the British about a settlement far short of a republic. All operations should have been suspended during those talks."

Coulthart says meeting Gorman helped the healing process for the Melroses: "At the start, the body language was telling. Roy Melrose, Stephan's father, looked at Tommy with hostility and contempt. After four hours of talking, that changed. Tommy faced hard questions about the murders. It is to his enormous credit that he replied candidly, and to Gerry Adams' shame that he didn't do the same.

"Tommy told the Melroses about civil rights protestors being beaten off the streets, about Catholics being unable to get jobs, about Bloody Sunday. It gave them a perspective on why young people had joined the IRA."

As the family were leaving Gorman's home, Beverley Melrose – Stephan's mother – admired the roses in his garden. The ex-IRA man picked her some flowers. "Beverley kissed and hugged him. It was extremely emotional," says Coulthart.

There were 67,000 British soldiers based in Germany. From its viewpoint, the IRA's European campaign was disastrous. It killed 10 people in 12 years – four of whom were civilians.

In September 1989, Heidi Hazell (26), the German wife of a British soldier, was shot dead by mistake. The following month, an RAF man of Indian origin, Maheshkumar Islania (34), and his six month-old daughter died in a gun attack. The AK47 used was the same one which killed the two Australians nine months later. Islania's wife escaped injury in the shooting. An eyewitness said: "It was a horrendous scene. She was clutching the dead baby, refusing to let it go."