The ETA – "Euskadi ta Askatasuna," or "Basque Country and Freedom" in the Basque language – has waged a violent struggle for independence for 40 years. ETA has been labeled a terrorist organization, and Spanish Prime Minister Jose Luis Rodriguez Zapatero called the arrest "another important step in the victory of democracy against terror," reports the Associated Press.
The Washington Post reports that two more ETA suspects were captured Wednesday, signaling a broader crackdown against the underground group. While the Spanish government singled Mr. Lopez Peña out as ETA's leader, it cautioned that the group was still dangerous, reports the BBC.
Spain's Interior Minister Alfredo Perez Rubalcaba hailed the arrest. He said Mr Lopez Peña was "in all probability, the person who carried the most weight within Eta - politically and militarily".
But he emphasized that the group remained a threat.
"This is a very important operation, because it involves very important leaders, so it should have a big effect. But I insist that Eta could still cause a lot of harm," he said.
ETA had claimed credit for two recent attacks. On Monday, a car bomb ripped through Bilbao, the capital of Basque Spain, damaging several buildings. No one was injured in that attack. On May 14, a bomb attack killed one policeman and injured four others in Legutiano, also in the Basque region.
The Marxist-Leninist ETA has long struggled to create an independent Basque nation along the border between Spain and France. Its attacks have killed more than 800 people since 1968, mostly in car bombings and shootings, according to Reuters.
Britain's The Daily Telegraph reports that Thierry may have taken the reins of ETA in 2006 and was key to the group's decision to end a cease-fire announced that year and turn its back on a budding peace process.
Peña, who has used the alias Thierry and been on the run since 1983, is suspected of being the mastermind behind a series of recent attacks that began with a car bomb at Madrid international airport in December 2006 that killed two people and brought an abrupt end to a fledgling peace process.
He is believed to have taken over Eta's underground leadership in 2006 when the group was holding peace talks with the government of Mr Zapatero. According to Spanish media, Peña, 49, participated in the talks but then decided to end the ceasefire.
The Spanish newspaper Publico writes that Peña once said "They will never catch me," but was wrong to think that his security measures and claim to French nationality would protect him, according to the Spain Papers Review at the website TypicallySpanish.com.
ETA are considered terrorists by Spain, America, and the European Union. ETA activists have long used South-western France as a base, while attacking targets over the border in Spain. In December last year, two young Spanish officers working undercover in France were shot dead by ETA suspects in a cafe on the French Atlantic coast.
Agence France-Presse reports that the three other suspects captured with Thierry were: Jon Salaberria, Igor Suberbiola, and Ainhoa Zaeta Mendiondo. The Spanish interior minister said all were "important leaders" of ETA.
Salaberria, a former regional lawmaker for ETA's now-banned political wing Batasuna, has been accused of financing the Basque separatist movement.
Suberbiola was a member of a Basque independence youth movement close to ETA before going underground.
Ainhoa Ozaeta is believed to be the masked woman who read a statement in an ETA video last year that officially called off a permanent ceasefire announced in March 2006.
Prime Minister Jose Luis Rodriguez Zapatero said he told the head of the Basque Country regional government, Juan Jose Ibarretxe, that the plan violated the Spanish constitution, during a meeting which took place as Basque separatist rebels ETA stage a bombing offensive.
The scene is now set for a confrontation later in the year if Ibarretxe tries to push ahead with the referendum in which he hopes Basques will vote to authorise talks between local political parties on the region's future.
The issue could dominate regional elections due by next year.
Compared with their conservative rivals, the Popular Party, Spain's Socialists are seen as more tolerant of autonomy drives in both the Basque area and Catalonia.
Significantly, as well, the Socialists scored the first-ever majority win by a national party in Catalonia and the Basque area – regions where local parties seeking greater autonomy or independence have long been most influential. The Socialist scores in these two most vibrant economies in Spain – whose capitals are Barcelona and Bilbao – suggest that the party's policies of gradually greater autonomy, much criticized by the Popular Party, may have gained traction.