On Christmas Eve 1981, an 18-year-old girl from the Spanish town of La Línea de la Concepción, just over the border from Gibraltar, was admitted to hospital. She was young, pregnant and unmarried – sinful in the eyes of a conservative society only just emerging from 40 years of dictatorship.

María Mercedes Bueno never saw her baby. The doctor put her to sleep during labour. When she came round, he informed her that her baby had been stillborn and that the hospital would take care of the burial. Twenty-eight years later, she discovered the truth: she and her daughter were among the tens of thousands – some estimate hundreds of thousands – affected by Spain’s “stolen children” scandal, an eugenics-inspired programme which had its origins in the early years of Franco’s rule.

Newborn babies were taken from mothers deemed undesirable and placed with pro-regime families. The initial targets were left-wing mothers imprisoned after the Spanish civil war. Over decades, the practice is thought to have evolved to target poor families, single mothers and others considered to be “morally unfit”. The kidnappings required the complicity of doctors, nuns, priests and government officials, and are suspected to have continued long after Franco died and democracy came to Spain.

When news of the scandal broke in 2010, I had just had my first child and the terrible crime shook me to the bones. In 2012, I moved my family back to my native Spain to investigate. Thus began a seven-year journey, with co-director Robert Bahar, of filming María and dozens of others.

These victims and survivors are seeking justice under universal jurisdiction, which allows foreign courts to investigate crimes against international law, such as torture, if the country where they occurred refuses to do so. Argentinian judge María Servini de Cubría took on the investigation, which has become known as the “Argentine Lawsuit” and could lead to prosecution in an Argentinian court. Ironically, it was Spain that pioneered universal jurisdiction in pursuit of former Chilean dictator Augusto Pinochet: now it is an Argentine judge who must bring Spain’s own past to light.

The plaintiffs’ long journey forms the backbone of our documentary film, The Silence Of Others.

The Argentine Lawsuit covers a multitude of crimes committed during Franco’s 40-year reign and its aftermath, and brings together families looking to be reunited in suspected cases of stolen babies, children and grandchildren seeking to re-inter the remains of loved ones killed by the regime and buried in mass graves, victims of torture seeking to prosecute still-living torturers, and many other crimes.

“Chato” Galante is one of the plaintiffs. Galante was tortured as an activist in the early 1970s. His alleged torturer is a notorious figure nicknamed “Billy the Kid” (Antonio González Pacheco), who lives in his neighbourhood – on a generous state pension that includes bonus payments as a result of several medals of honour.

Like many of the people we filmed for years, Galante is haunted by the urgency of Servini’s investigation. “You don’t understand what it’s like,” he says, “to talk to someone who’s 80 who tells you ‘I need to talk to the judge because I may die soon’.” Last week, he went with a group of torture survivors to Brussels in the hope of convincing the European parliament to investigate the obstacles that prevent the Spanish government from prosecuting these crimes.

Others, such as María Martín, 82, were merely fighting to give their loved ones a dignified burial alongside their families. Martín’s mother was a civilian executed in the early days of the civil war, and buried in a mass grave, now covered over by a motorway. “How unjust life is,” she lamented, sitting by the roadside, as close to her mother as she could, before correcting herself. “Not life. We humans … we are unjust.”

Servini’s investigation has led to requests to extradite and interrogate alleged torturers and former regime officials, to which the Spanish government has not agreed. Other requests, though, including the historic exhumation of labour activist Timoteo Mendieta that Servini ordered, have taken place. This month there have been several major developments, and glimpses of what justice might look like for the thousands of Franco’s still-living victims.

Two weeks ago a doctor, Eduardo Vela, who is implicated in “stolen baby” cases, was found culpable in a Madrid court of the kidnapping of Inés Madrigal, taken for adoption in 1969. Days later, the relatively new socialist government said it would not stand in the way of Servini’s inquiries, including into the shooting of civilian aactivists in the year after Franco’s death. Among others, Servini seeks testimony from Rodolfo Martín Villa, who was then interior minister and became a deputy prime minister in the democracy that followed.

And minister of the interior Fernando Grande-Marlaska has announced moves to strip González Pacheco of his service medals and his topped-up pension. But these advances are fragile at best: Vela was found guilty but was still acquitted – the court claiming the statute of limitations had passed. Earlier this month, “Billy the Kid” was spotted at an official reception at a Madrid police station, and two attempts to file torture cases against him in local courts were dismissed as outside the statute of limitations.

There were angry and emotional reactions to these developments from activists and from torture victims, who pointed out that, under international law, there is no statute of limitations for crimes against humanity.

Meanwhile, though the state may have signalled its willingness to co-operate with the investigation, ultimately it will be up to the courts whether they wish to play ball. In the past, Spain was prevented from confronting its trauma by its 1977 Amnesty Law, part of the “pact of forgetting” under which its leaders agreed that neither perpetrators nor victims would see justice. The law was seen as necessary to prevent score-settling in post-Franco Spain. But while political prisoners were freed (though their convictions stand to this day), it also meant that those who had signed execution orders, murderers, torturers and kidnappers, or dumped human remains in mass graves, were never held to account.

Not so long ago, former prime minister José María Aznar underlined the argument for forgetting in a speech: “The spirit of harmony, born in the Spanish transition, is not made by stirring up bones.”

However, the truth is that for too many there was never harmony, and – until these crimes can be addressed – there will not be peace.

Now, as new generations learn about these chapters in our history, and as measures to address these issues are perhaps on the horizon, it feels as if any consensus around our national amnesia is breaking down. We can’t change our history and we may never agree about it completely, but steps can be taken to redress the injustices of the past.

The victims and survivors who have spent decades battling the silence continue to seek measures towards truth, justice, and reparation of crimes that took place. A voluntary DNA database could be established to help families find each other, should they wish to; the government could help those who want to exhume loved ones’ bodies from mass graves and re-inter them with family members. But, most importantly, the plaintiffs hope that the government and the courts will either remove the obstacles to pursuing these investigations in Spain, or cooperate with Servini in her investigations, which could be the only chance of justice for many who have suffered.

As María Bueno told us: “We’re asking for truth and justice. It’s all we have left.”

Almudena Carracedo is an Emmy-winning film-maker. Carracedo and Robert Bahar’s film, The Silence of Others, will screen in London at Bertha DocHouse on 27 October and will be released in Spain on 16 November