The six, who were arrested on May 7th in a sweep Spain said had brought ETA close to being wiped out, were remanded in custody, the source said.
They face charges of "financing terrorism", "criminal conspiracy linked to terrorism" and are also being investigated for possessing arms and fake documents.
Spain's Interior Minister Jorge Fernandez Diaz had said the arrests were "a very important step towards the dismantling of ETA", listed as a terrorist group by the European Union and United States.
The group is blamed for 829 deaths in a four-decade campaign of bombings and shootings for independence for the Basque Country of northern Spain and southwestern France.
It declared in October 2011 a "definitive end" to its armed activity but has not formally disarmed or disbanded as the Spanish government demands.