In recent years a determined, Anglo-Chilean woman has become a common sight in the streets of Valparaíso, the port city that was the power base of General Augusto Pinochet. She has spoken to judges, lawyers and ordinary Chileans. Gradually, in a remarkable tale of courage and perseverance, she has pieced together the grisly events that took place over 30 years ago, when she lost her brother to the brutality of the Pinochet regime.

Nineteen former naval officials have just been arrested as a result of Patricia Bennetts's efforts. Four vice-admirals, several captains and other Chilean Navy officers face being put on trial. More importantly, justice appears finally to be possible for her brother Michael, a young priest who fell foul of one of the most notorious dictatorships in 20th-century history.

Michael Woodward was an idealistic young Englishman, a Roman Catholic priest swept along by the hopeful currents of a new radical 'liberation' theology. He saw his future as helping the poor raise themselves from poverty through the power of prayer and politics. It was a vision that took him back to Chile in the early Seventies, the country where he was born in 1932.

There he began to work and live among the people of Cerro Placeres, a working-class neighbourhood of Valparaíso. It was a world away from his privileged childhood as a British public schoolboy. Chile was undergoing a political transformation after the election of the charismatic new Socialist President, Salvador Allende, in 1970. General Augusto Pinochet and his military henchmen brought that experiment in Latin American socialism - and Woodward's ministry to the poor - to an abrupt and violent end. Within 10 days of the 1973 coup, Woodward had disappeared. Eventually his family heard he was dead, though no one could tell them how, why or where his body was buried.

Thirty-five years later, as a result of the indefatigable efforts of his sister, that wall of silence is being demolished. Woodward's younger sister, Patricia, a mild-mannered 70-year-old, has returned to Cerro Placeres and set about finding the answers to the questions no one would provide all those years ago.

In a country where the deceased dictator still casts a dark and occasionally sinister shadow, she has encountered intimidation and fearful silences. 'The investigation has uncovered a lot of things,' she said. 'Other cases have come to light. It has been discovered that all this was planned before the coup.'

Death threats, break-ins, thefts of documentation and angry demonstrations by Pinochet supporters have all formed part of a concentrated attempt to intimidate Bennetts and those helping her. However, her single-minded pursuit of those who kidnapped, tortured and killed her brother has borne fruit.

Supported by justice officials operating in an increasingly liberal atmosphere under Socialist President Michelle Bachelet, herself a torture victim, Bennetts has uncovered many of the painful and tragic details of her brother's fate. Father Michael Woodward was almost certainly shot dead. A bullet was fired into his chest - possibly while being held on an elegant, four-masted navy training ship, the Esmeralda. Two witnesses have spoken of a chest wound. His death followed 10 days of brutal torture.

The revelation places extra pressure on the investigating judge to bring those arrested to trial. That judge, Eliana Quezada, regularly receives death threats and needs a police bodyguard. 'Judge Quezada has been receiving death threats for a long time. In recent times it has got a lot worse,' Patricia Bennetts said. 'I think it is because of this case.'

A state human rights lawyer helping prosecute the case, Karina Fernández, recently had her house broken into and her laptop computer, containing documents about the case, stolen. 'She was away from home for several hours and somebody got in and stole it,' Bennetts explained. 'They purposely left her jewellery, money and cheque stubs on her bed as if to say, "This is the only thing we wanted".'

Patricia Bennetts has also needed police escorts at Valparaíso's courthouse as groups of well-heeled, jeering, pro-Pinochet protesters jostled her. 'It has been very hostile,' she said. 'One lady tried to kick me.'

Patricia and Michael were born to an English father and Chilean mother. They spent their childhood in Latin America while their father worked as a manager for British American Tobacco. They were sent home to Britain to attend school. Michael went to Downside, a Catholic public school in Somerset. He studied engineering in London but eventually returned to Chile, became a priest and ended up in Valparaíso, a navy town north-west of Santiago.

Liberation theology swept through Latin America during the Sixties, as many Catholic priests decided their faith required a commitment not just to God but also to social justice and political action. Michael Woodward was among them and, by the early Seventies, he had joined a Christian Marxist group called the Movement for United Popular Action, or MAPU. The group supported Allende when he came to power in 1970, amid hopes that he would transform Chile. Woodward had already been suspended by a conservative church hierarchy when Chile violently lurched from socialism to right-wing dictatorship.

In September 1973 the army commander General Pinochet rebelled, together with other military and police chiefs. Allende and a handful of supporters held out briefly in Santiago's La Moneda palace, but were surrounded by tanks and attacked from the ground and the air. Allende is believed to have turned his own gun on himself as troops entered the palace. The dream of a socialist Chile died with him.

Pinochet, a man whose ruthlessness, dark glasses and German-style military uniforms helped earn him a reputation as the world's archetypal Latin American despot, took control and stayed in power for 17 years.

It was only after Pinochet stepped down as head of the armed forced in 1998 that Chileans began to hope that they might seek justice for crimes committed in his name. 'Before, people could not speak without fearing for their lives,' Bennetts explained.

Pinochet's 1998 arrest in London, after Spanish magistrate Baltasar Garzón requested his extradition, raised hopes further - although legal arguments over his British victims concentrated on the killing of stockbroker William Beausire and the torture of British doctor Sheila Cassidy. His death two years ago, when he was being investigated for hundreds of deaths and corruption, removed many of the remaining obstacles. Even now, however, amnesty laws make it hard to get sentences passed for murder. 'The people who are being indicted are being held for kidnapping, not for murder,' said Bennetts. 'If they were indicted for murder, they could appeal to the law of amnesty and be freed.'

Bennetts's investigations have helped Chileans piece together the horror of what happened in Valparaíso in the weeks that followed the coup. Her lawyers have shown how navy units took control of the city and set up torture and detention centres in at least three places. These included the Navy War Academy, the Federico Santa Maria University and the Esmeralda.

She has also pieced together the terrible saga of her brother's detention, which saw him pass through all three places before, almost certainly, being shot and buried in an anonymous grave. Woodward was picked up ten days after the coup, when he returned to his house in Cerro Placeres. He went into hiding after the coup and naval units had already ransacked the house. It was, in retrospect, rash to go home, but Woodward insisted he had nothing to hide. Few people realised what was happening to those picked up by the military.

His ordeal began almost immediately. Witnesses now say that they saw him at the university detention centre. There he was subjected to torture sessions in the swimming pool. 'There were witnesses to say he was sunk and lifted inside the pool,' said his sister.

From there he was taken to the Navy War Academy. 'That is where he got most of the torturing,' Bennetts said. 'He was tied to a chair and tortured for an hour with lots of people looking on to see how it was going.' Torturers wrapped their fists in damp towels so that the beating left fewer external marks.

He was eventually taken to the Esmeralda. 'There was violence 24 hours a day. Prisoners were taken out, beaten and tortured, returning bruised and vomiting blood,' a former detainee, María Aliane Comene, said. 'They took me out every night to interrogate me. They hit me on the ears with their hands, they applied electric currents to my tongue and my vagina. They took us out to amuse themselves, to abuse us sexually. They raped us.' Sergio Vuskovic, a former Valparaíso mayor, was tortured for seven days. 'They applied electrical discharges to my penis, my testicles, my torso and my back,' he said.

Captain Carlos Fanta, the senior naval officer in Valparaíso on the day Woodward died, has admitted that a navy doctor ordered his evacuation from the Esmeralda after certifying that he would die of the internal bleeding caused by his beatings. By then, Chilean navy officials had planted a newspaper story claiming that the 'pseudo-priest took part in various attacks on police... and sexually abused an indeterminate number of young girls'. On the same day a death certificate was signed, saying Woodward had died of a 'cardio-respiratory arrest' on a road.

Reaction to the case has been mixed. 'We have more important things to worry about than stupidities like this,' said Ricardo Bustamante, the head of the students' union, when told that the private Federico Santa Maria University had been used as a torture centre.

Carlos Portales, a lawyer representing the accused, used a recent celebration of the Pinochet coup 'and the things that happened afterwards' to claim that the courts were now in Marxist hands. 'With Marxists there is no way of reaching an honourable understanding,' he said. 'We need to change the way we act. We have been too passive.'

For years, Patricia Bennetts has campaigned to stop the Esmeralda, also known as The White Lady, being allowed to visit ports in Britain or elsewhere in the world until the Chilean Navy publicly recognised and apologised for its use as a torture centre. However, it was not until 2004 that the navy accepted that 100 prisoners were tortured and raped on board. Navy officials still claim today that only a minority of officers and sailors were involved.

Bennetts is now satisfied that many of those involved in the arrest and torture of her brother have been found. She hopes that sentences will be passed before the end of the year.

She and her husband, Fred Bennetts, now spend much of their time in Cerro Placeres among the people her brother had tried to help. 'It is encouraging to see how people in the street react,' she said. 'They stop and ask us how it is going. We feel their support.'

The final piece of the puzzle, however, has yet to be put into place. Woodward's body is thought to have been sneaked into the city's Playa Ancha cemetery at night by naval personnel. Two years ago Bennetts persuaded Judge Quezada to dig up a section of the cemetery after a tip-off. Her brother's corpse was not found, however.

Now the family is in possession of new information. A former grave-digger has said that he was forced at gunpoint to bury three men together, all of whom had been shot in the chest. 'The location of the grave has been pinpointed within a relatively small area,' said Fred Bennetts. 'The area is just a few metres away from another grave in which a skull with a bullet wound was found next to a bullet casing of a calibre used only by the armed forces. I saw the skull and the bullet casing myself.'

For the moment, Patricia Bennetts is concentrating on the court case. She says that she will not be put off by the small crowd of former naval personnel who try to intimidate her at the courthouse doors. Nor, like her brother before her, will she respond in kind.

'We will keep our cool,' she said. 'We certainly won't shout.'

The Dictator and the Priest

1915 Augusto Pinochet born in Valparaíso, Chile.

1932 Michael Woodward born in Chile.

1947-1953 Woodward attends Downside school, then studies engineering at King's College London.

1954 Woodward returns to Chile and enters a seminary.

1961 Woodward ordained.

1970 Socialist Salvador Allende becomes President of Chile. Woodward has joined the Marxist Christian MAPU group.

1973 Allende dies in coup led by Pinochet. Woodward tortured and killed by naval personnel in Valparaíso.

1990 Pinochet stands down as President but remains commander-in -chief.

1998 In March Pinochet steps down as commander-in-chief. Arrested in London on a Spanish court order in October.

2000 Home Secretary Jack Straw sends Pinochet home in March on grounds of ill-health.

2006 Pinochet dies days after being stripped of immunity to prosecution.

2008 Nineteen former naval personnel charged in Woodward case.