Ternera was arrested Thursday in the town of Sallanches. He was convicted in absentia in 2017 in France for involvement in a terrorist group and sentenced to eight years in prison. He has the right to request a new trial.
Spain says it will ask France to extradite Ternera to stand trial for alleged crimes against humanity and multiple killings before he completes a French sentence. ETA, whose initials stand for "Basque Homeland and Freedom" in the Basque language, killed more than 850 people during its decades-long violent campaign to create an independent state in northern Spain and southern France. The militant group gave up its arms in 2017 and disbanded last year after being weakened by efforts to dismantle its operations and arrest its leaders.
Ternera was one of the negotiators who sat down with Spanish government envoys for talks to try to end the group's activities in the mid-2000s. His voice was identified as one of two ETA members who read a statement announcing the group's dismantling on audio recordings released on May 3, 2018.
Mayor Jean-Marc Peillex of Saint-Gervais-les-Bains said Ternera had been housed in the village of Saint Nicolas de Veroce, on the outskirts of his town. He speculated Friday that police were searching for accomplices who helped hide the long-time fugitive.
"Who helped him? Who were the people around him? Were they aware? Were they not aware?" the mayor said. He also said an area popular with tourists is easy to hide in. French President Emmanuel Macron, touring the French Basque city of Biarritz ahead of a G-7 summit, was asked about the possibility of freedom for Ternera, whose real name is José Antonio Urruticoetxea Bengoetxea.
He replied "Political reconciliation does not mean amnesty."
Jamey Keaten in Saint-Gervais-les-Bains contributed.