According to the officials, the political leader of the Basque separatist group Eta has been arrested in the French Alps after 17 years on the run.
In a joint French-Spanish police operation, Josu Ternera - whose real name is José Antonio Urrutikoetxea - was arrested in Sallanches.
He is condemned over several Eta killings.
Eta, which waged a violent campaign for Basque independence for more than 40 years, said in 2018 it was disbanding.
The group killed more than 800 people during its campaign of violence before declaring a ceasefire in 2011.
According to the reports, Ternera is seriously ill and Spanish police tipped off their French colleagues that he was on his way to the hospital, triggering his arrest.
In an audio announcement in May 2018, Ternera said Eta had "completely dismantled all of its structures" and would "no longer express political positions, promote initiatives or interact with other actors".
He said former members would "continue the struggle for a reunited, independent, socialist, Basque-speaking and non-patriarchal Basque Country".