The ex-police officer from Argentina, who has lived for more than three decades in France, was extradited for crimes during the military dictatorship.
France has extradited an Argentine ex-police officer linked to the murder of hundreds of people during the country's 1976-83 military dictatorship.
Mario Sandoval was sent on a plane that left Paris around midnight on Sunday to face trial in Argentina over the disappearance of a student.
He was arrested on Wednesday at his home near Paris, after French authorities gave the final go-ahead for his extradition.
The 66-year-old had been living in France since 1985 and obtained French citizenship with few aware of his past.
Argentina suspects that Sandoval took part in more than 500 cases of kidnappings, torture and murder at a time when some 30,000 were "disappeared" during the dictatorship.
But the extradition concerns only the alleged kidnapping in October 1976 of Hernan Abriata, an architecture student whose body has never been found.
8-year legal battle
Sophie Thonon, a lawyer acting for Argentina, told Euronews that the legal battle to extradite Sandoval took 8 years — the time needed to exhaust all legal remedies available to him.
The French Council of State, which advises the government on legal matters, approved his extradition in August 2018, prompting Sandoval to appeal.
The Constitutional Council determined that no statute of limitations could be applied to an "ongoing" case, citing the fact that Abriata's body has never been found.
Sandoval's lawyers had argued that he would not get a fair trial in Argentina where he would face torture or poor detention conditions.
But their appeals to the European Court of Human Rights to take up his case failed.
Thonon said the argument that Sandoval risked facing a political trial if he returned to Argentina was used by his lawyers in their attempt to prevent his extradition.
"Of course, this is in no way a political trial," Thonon said. "Sandoval is wanted because he is suspected of crimes against humanity. "
"I hope consular officials... will ensure that the conditions of his detention pending trial will be decent and limited in time," Sandoval's lawyer Jerome Rousseau told AFP.
Abriata's family seeks justice
Abriata was detained at the notorious ESMA navy training school in Buenos Aires, where an estimated 5,000 people were held and tortured after the military coup of 1976 -- many of them thrown from planes into the sea or the River Plate.
Thonon told AFP that Abriata's 92-year-old mother Beatriz Cantarini de Abriata had been "desperately waiting" for Sandoval to "explain himself before Argentine justice".
Sandoval, who has dismissed the accusations as fabrications, fled Argentina after the military junta fell.
Sandoval was a professor at the Sorbonne's Institute of Latin American Studies in Paris and the University of Marne-la-Vallee outside the French capital.
In France and Argentina, many are left wondering how the former torturer could pursue an academic career for decades without any questions being asked.
But Thonon noted French Universities pushed him aside when his past became known in 2008.
"He has always dissimulated his role to present himself in the best possible light," the lawyer told Euronews. She added that she had reviewed his resumes and that "none of them" looked alike.
His colleagues at both schools called for his arrest when they recognised his picture during his legal battles.