Txeroki is suspected of killing two police officers in December 2007
The suspected head of Basque separatist group Eta has been placed under formal investigation after his arrest in France this week, judicial sources say.
The step gives investigators more time to examine evidence.
Mikel Garikoitz Aspiazu Rubina is being held on suspicion of "association with a terrorist enterprise", sources said.
He was detained in south-western France on Monday, and is a suspect in the murder of two Spanish Civil Guard officers in France in 2007.
After his arrest, Spanish authorities described Mr Garikoitz Aspiazu, 35, as the suspected military head of Eta.
KEY ETA ARRESTS
November 2008: 'Military leader' Mikel Garikoitz Aspiazu Rubina held in south-west France
May 2008: 'Political leader' Javier Lopez Pena held in Bordeaux
July 2007: 'Logistics chief' Juan Cruz Maiza Artola detained in Rodez, southern France
October 2004: Long-time 'political leader' Mikel Albizu arrested in Salies de Bearn, southern France
December 2002: 'Military leader' Ibon Fernandez Iradi held in southern France
However, Interior Minister Alfredo Perez Rubalcaba has since said that he was actually Eta's top leader, in charge of overall strategy.
Mr Garikoitz Aspiazu - known by the nickname Txeroki, or Cherokee - allegedly took over after the arrest of Eta's suspected political commander, Javier Lopez Pena, in May.
"Txeroki ended up in charge of everything - the political apparatus, the so-called military apparatus. The one who ordered killings was Txeroki," Mr Perez Rubalcaba said.
Eta is blamed for the deaths of more than 820 people in its 40-year campaign for an independent Basque nation.