A former police officer from Argentina who has lived for more than 30 years as a university lecturer and security expert in France was extradited and arrived on Monday in his native country, where he is wanted for crimes against humanity, including torture, committed during the country’s dictatorship, the French lawyer representing Argentina said.

The news of the extradition of the former police officer, Mario Sandoval, was welcomed in Argentina by human rights activists and relatives of victims of the military regime, which killed up to 30,000 people, according to estimates, between 1976 and 1983.

But even as Mr. Sandoval was arrested at his home in a Paris suburb on Wednesday, many in France tried to fathom how a suspected torturer was able to live there for decades, teaching international affairs, participating in security conferences on Latin America and working with French security officials.

Prosecutors in Argentina have accused Mr. Sandoval, 66, of having participated in the death of an architecture student, Hernán Abriata, who disappeared in 1976 at a notorious secret detention center in Buenos Aires.