The operation was made possible by the arrest in France on Sunday of 20 ETA suspects including the group's alleged leader, and in northern Spain of a truck driver suspected of involvement in recent bombings of electricity pylons, Alonso said. "Police believe the people arrested had agreed to join the terrorist group's operative structure, and were awaiting instructions (to carry out attacks)," the minister told a news conference. "Documents and computers have been seized and the operation is ongoing as these arrests are linked to information extracted in last weekend's operations in France and northern Spain," he said. The arrests of four men and one woman took place in the early hours of the morning in the northern provinces of Guipuzcoa and Navarre. Spain regards Sunday's extensive operation by French police, who nabbed suspected ETA political leader Mikel Albisu Iriarte, alias "Mikel Antza," as the biggest coup against the separatist group in 12 years. French police also uncovered huge ETA arms caches, including a ton of explosives, grenades and launchers, assault rifles and two Russian-made surface-to-air missiles. "We have to maintain maximum political, police and judicial pressure (on ETA) and not drop our guard for a single moment," Alonso said. "I want to insist on that in order to avoid euphoria, though it would be justified by the good news. Politicians and society in general have to understand we have to remain cautious and keep working."
ETA, classed as a terrorist group by the United States and the European Union, is under pressure on both sides of the Pyrenees after the wave of recent arrests. The group has killed more than 800 people since 1968 in a campaign to press for a Basque state carved out of northern Spain and southwestern France. However, it has only carried out small-scale bombings recently and has not staged a fatal attack in 16 months. Analysts say it has been weakened by hundreds of arrests in France and Spain in recent years.