Investigations are continuing in France following the arrest of ETA suspects this weekend.
Madrid believes two key ETA members are among the three men and a woman being held after a raid on a house police have described as a “bomb factory”.
They say they have found 30 detonators, among other objects.
Separatist slogans were heard coming from the house near Cahors when the detainees were taken back there as a detailed search went into a second day.
Spain claims all four played a “decisive role” in the December bombing of a car park at Madrid’s Barajas Airport.
The blast, which claimed two lives, prompted Socialist Prime Minister, Jose Luis Rodriguez Zapatero to break off peace talks with ETA.
He has headed a crackdown on the group since it officially ended a ceasefire in June.
Addressing a political gathering he said: “You know I’ve put all my energy into ending violence through dialogue. ETA has chosen violence so I repeat today what I have said before, we will be unrelenting faced with violence and with bombs and with those who use them.”
Zapatero’s popularity has risen since the crackdown. His party now has a three and a half per cent lead over the opposition in polls with elections due in six months.