"Did my child die or was he kidnapped?" is something no parent should ever have to ask, and still less so when the kidnappers are the government. But that is exactly the question hundreds of Spanish families are currently demanding that their courts resolve for once and for all about the so-called "lost children of General Franco". They were already estimated to total around 30,000, and now, it appears, there may be many more.

In Franco's early years, "child-stealing" by the Spanish state was politically motivated, with its key instigator, Antonio Vallejo-Nagera, the army's crackpot chief psychiatrist who championed Nazi theories that Communism was a mental illness caused by the wrong kind of environment. Inspired by Vallejo-Nagera, Franco's government passed laws in 1940 that, as one judicial report in 2008 put it, "ensured that families that did not have ideas considered ideal [ie, supporters of Spain's defeated republic] did not have contact with their offspring".

Putting this policy into practice was brutally straightforward and efficient. In 1943, records show 9,000 children of political prisoners had been removed to state-run orphanages, and in 1944 that total had risen to more than 12,000.

Arguably the most infamous case took place at the Saturraran women's prison in the Basque country, when around 100 Republican children were removed in one fell swoop. Their mothers, who had been tricked into leaving their children alone for a few minutes, were told they would be shot if they so much as shouted when they came back and found them gone.

Julia Manzanal, 95, no longer talks to the press because her family say that it upsets her too much. But as a Communist whose 10-month-old baby died of meningitis in one of Franco's prisons she was a first-hand witness of the enforced adoption policy. When last interviewed in 2003 she said : "I never let my child out of my sight because when mothers were condemned [to death], they would rip the babies out of their arms. They would give them to priests, to military families, to illegal adoption rings and educate them in their own ideology. Conditions there were terrible... there were huge rats, lice, virtually no food, women would give birth in the washrooms with no help... I saw children die of hunger and thirst, and their mothers would go mad as a result."

Having the wrong name could be fatal. In a television documentary in 2002, Ms Manzanal described how when Franco's police discovered that one prisoner's child's name was Lenin, they picked it up by the legs and smashed its head against a wall.

Even after the collapse of Nazi Germany, the enforced adoption policies continued, and even intensified to include Republicans living abroad. As late as 1949, official documents of the ruling Falange party give detailed instructions on how children born to their former enemies then exiled outside Spain were to be kidnapped and brought back across the border for re-education. Their names were then changed to ensure no further contact was possible.

But by the 1960s what had begun as a politically motivated state policy slowly morphed into a more straightforward adoption trade – in some cases with the state's connivance. Parents were simply told their infants had died shortly after birth, and the babies were then sold on to families.

Mar Soriano told El Pais newspaper last year: "My sister was born on 3 July 1964, and my mother was breastfeeding her until they told her they had to take her baby to the incubator. When my parents went to look for her later, they told them she had died of an ear infection. My father wanted to see her and bury her, but they said they had taken care of everything and she was in a mass grave."

Other cases, like that of Maria Jose Estevez, were eerily similar. Ms Estevez's baby was born on 3 September 1965 in Cadiz, but even though she could hear him crying later in the next room, she was told she was imagining things and that he was dead. She was informed he had already been buried, next to the amputated leg of a recently operated patient.

With cases now up to six decades old, any hope of resolving them seemed doomed. But a recent wave of media interest has seen bereaved family after bereaved family recalling the same bizarre circumstances: the death of their newborns from ear infections or an equally implausible cause, followed by the hospital's point-blank refusal to show them the body.

By late November, Javier Zaragoza, Spain's chief prosecutor, had more than 300 new cases on his desk. Faced with growing demands, he formally requested that the Ministry of Justice set up a specific department to compile a list of the missing infants.

However, there was a catch. Mr Zaragoza was willing to run the investigation to cover a massive four-decade period – up until 1980, five years after Franco's death – but he also said that it would be purely administrative. In other words, even if crimes were uncovered, nobody would go to jail.

Discouraging as that may sound, it represents progress compared with 2008, when the first official report made into the cases of all the "disappeared" during the Franco years ordered by the crusading judge Baltasar Garzon, including the missing infants, ended up being shelved. Judge Garzon was accused by various extreme right-wing organisations of acting outside his legal powers, something for which he now faces trial.

This time round, though, the victims of enforced adoption are determined that they will not be shunted into a legal siding and forgotten. So far, they are succeeding. In Madrid, the hospitals have opted for a full-scale investigation of all infant deaths between 1961 and 1971.

In Cadiz, Algeciras, Malaga and Granada, four big cities in the south, the local state attorneys are reported to believe cases should be opened. In Valencia, a leading lawyer specialising in the cases, Enrique Vila, aims to open another legal front later this month when he files a formal complaint of mass kidnapping with Spain's equivalent of the Crown Prosecution Service.

There could even shortly be an international investigation. The Foros por la Memoria movement has taken the cases of all those missing from the Franco years to the United Nations to plead that they cannot simply be shelved. An answer is expected this summer.

As for the women of Saturraran prison, last year, for the first time, a film, Izarren argia [now Stars to Wish Upon], was made about their experiences. When it had its premiere at the San Sebastian Film Festival, a 93-year-old former internee, Ana Morales, stood up in the audience and thanked the director for "finally letting some light be shed on that terrible place".