Senior ETA leader Mikel Irastorza, who was arrested over the weekend in southwest France, was transferred to Paris on Monday for questioning, judicial sources said.
Irastorza, deemed the topmost leader of the weakened Basque separatist group still at large until his arrest on Saturday, was set to be questioned by an anti-terrorism judge, the source said.
The judge will also interview a couple who allegedly sheltered him: 59-year-old Basque exile Xabi Arin Baztarica and his 56-year-old partner Denise.
Irastorza, 41, was found in a home in the French town of Ascain, in the Pyrenees region bordering Spain, in a raid led by French security forces working with Spanish police.
Founded in 1959, ETA waged a violent decades-long campaign for an independent Basque homeland in northern Spain and southwestern France, and is blamed for the deaths of more than 800 people.
It declared a ceasefire in October 2011 but has refused to give up its weapons.
The group is seeking to negotiate its dissolution in exchange for amnesties or improved prison conditions for the roughly 350 ETA members held in both countries.