Gil Carbajal in Madrid reports the operation took place as Spain's two major political parties discussed a common strategy in the face of the terrorist organization's current campaign. They also searched three apartments in Madrid and one in a nearby town.
Police say they discovered false identity documents and weapons.
There was also evidence indicating the suspects came to Madrid to gather information for new terrorist attacks. They are not believed to have participated in a car bomb attack one week ago which killed a Supreme Court judge, his driver, and his bodyguard. ETA has not yet claimed responsibility for that attack, but it has admitted setting off three other car bombs in Madrid since it called off a 14-month truce in December. ETA is fighting for a separate homeland, and has been blamed for a series of bombings. Police also speculate the group arrested in Madrid was sent to replace those who carried out the most recent car bomb attack.