Preliminary technical work has resumed on exhuming the remains of four Spanish Civil War victims from a complex that pays homage to the late dictator Francisco Franco, Spanish heritage authorities said.

The four are believed to lie in crypts within the Valley of the Fallen, a controversial neoclassical mausoleum north-west of Madrid where thousands of victims from both sides of the 1936-1939 war are buried alongside Franco’s remains.

Franco presented the grandiose complex as a symbol for national reconciliation, but victims’ relatives and activists have been campaigning against it for years because they consider that it pays homage to the late dictator.

Some even took the issue to court, asking for the burial niches to be opened and remains to be handed over to victims’ families. They argued relatives were never informed about the burials and that Franco’s tomb in a prominent location by the basilica’s altar is an insult to the victims’ memory.

In a first, a judge in 2016 ordered the exhumation of the Lapena brothers, members of an unauthorised anarchist labour union who were executed without trial in 1936.

But the basilica’s abbot stopped the preliminary work for their exhumation for nearly two years by appealing the judge’s decision, until top Catholic Church authorities ordered him earlier this year to comply with the judge’s ruling.

Eduardo Ranz, the lawyer for Manuel and Antonio Lapena’s descendants, said that relatives of Pedro Gil and Juan Gonzalez, two soldiers who died fighting for Franco’s national army, have also requested their remains to be removed.

“Spaniards will be witnesses of this historic day,” the lawyer said of the work that started in the crypts on Monday. But he added that the final goal is for the relatives to be reunited with the deceased.

“That will be the end of the process, when the real reparation can finally be delivered.”

A team of architects needs to assess the state of the crypts and set the conditions under which the exhumation work can be performed, said a spokeswoman with Spain’s National Heritage administration.

She said it was uncertain how long the preliminary work could last.

A commission of experts looking into the future of the Valley of the Fallen recommended in 2011 that Franco’s remains should be removed from the basilica, but the experts left the final decision in the hands of the government.

A separate investigation also found that water leaks and dampness in chapels and crypts holding the remains of more than 33,000 war victims had turned some of the niches into “piles of bones”.