Police have now tracked down six hideouts packed with bomb-making materials, weapons and documents, after arresting three suspected ETA suspects in a dawn raid on a flat in the Alpine resort of Le Corbier on Wednesday.
One of those arrested was Alberto Machain Beraza, 28, whose picture was circulated after the Majorca attack, in which two Spanish officers were killed.
The others were named as Aitzol Etxaburu Arteche, 30, an alleged ETA leader, and Andoni Sarasola Yarzabal, 36.
The trio were transferred Friday to the Paris region where they can be held for questioning by anti-terrorist investigators for another 48 hours before being brought before a judge to face possible charges.
Hours after Wednesday's raid, French police found 112 kilos (245 pounds) of ammonium nitrate and 12 litres of nitromethane, used to make bombs, at an ETA hideout in Ferrieres, around 40 kilometres (25 miles) from the Spanish border.
The following day they found a cache in Camplong in the Herault region, inland from the Mediterranean and a few hours from the Spanish border, packed with enough explosives to make "dozens of bombs" according to Madrid.
Along with explosives, it contained 450 metres of fuse, 83 detonators, two stolen handguns with some 2,600 ammunition rounds, portable radios, police uniforms and paperwork, said Spain's interior ministry.
Two more ETA hideouts were found in the Herault region Friday, in the towns of Ferrals-les-Montagnes and Vieussan.
In Ferrals, police seized around 15 handguns, 2,000 rounds of ammunition, around 50 detonators and a supply of the powerful explosive PETN, or pentrite, said officials connected with the inquiry.
The hideout in Vieussan contained a bomb with a sucker pad for fixing under a vehicle, paperwork and other bomb-making materials.
A smaller cache was also found Thursday in Hellette in the Pyrenees, just across the border from Spain.
And on Friday, police found a sixth cache: 25 kilos of ammonium nitrate and 12 of PETN together with electrical equipment at Minerve, southwest France, near the border with Spain.
Officers also recovered two revolvers stolen during a robbery in October 2006, court officials in Paris said.
And according to a Spanish interior ministry statement Friday, French police discovered a seventh cache, but a French official close to the investigation said the information was not correct.
ETA is blamed for the deaths of 828 people in its campaign for an independent Basque homeland in part of northern Spain and southwest France.
It figures on several terrorist blacklists, including those of the European Union and the United States.
ETA has tended to concentrate its attacks on Spanish targets while its militants use France as a rear base to hide out and rearm. In recent years French officials have arrested several senior suspects.
"This is naturally a very positive sign in the struggle against ETA, and proof of the high quality of cooperation between French and Spanish authorities," said France's Interior Minister Brice Hortefeux.
Intelligence supplied by Spanish investigators helped lead French police to the arms caches, according to an official connected with the case, who described the operation as "a very good catch."
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