Police are looking for Antonio Troitino, who was convicted of killing 22 people in the 1980s and sentenced to 2,700 years in prison, the BBC reports.
Last week, despite angry protests from groups representing the victims of ETA's armed struggle, a Spanish court cleared Troitino for an early release after 24 years in prison.
The court ruled that the six years Troitino spent on remand could be deducted from the maximum of 30 years a prisoner, under Spanish law, can spend behind bars, the BBC reports.
Under pressure from victims' organizations and public prosecutors, the court has since revoked that decision, saying Troitino must go back to prison until 2017.
The problem is that Troitino wasn't placed under surveillance after his release -- police don't even know whether he's still in Spain, the BBC reports. The 53-year-old has been sentenced for his involvement in several bombings, including an attack on a police bus in Madrid that resulted in the deaths of 12 people.
During the past years, French-Spanish police cooperation has thinned out the top ranks of ETA, which has to rely on increasingly young and inexperienced personnel. Observers say the group is weaker than ever before.
The group last year via Basque newspaper Gara declared a permanent cease-fire to demonstrate its "firm commitment toward a process to achieve a lasting resolution and toward an end to the armed confrontation."
The pledge to stop attacking didn't convince Spanish authorities. Prime Minister Jose Luis Rodriguez Zapatero said ETA's announcement didn't go far enough. The group should renounce violence forever, he told Spanish television.
"Those who see some element of hope in ETA's announcement need to know that the road ahead is still long, because the only thing that matters is the definitive end of the ETA terrorist group," Zapatero said, adding that there won't be any negotiations with ETA. "We are not going to allow any trick," he said.
Madrid has reasons to be suspicious when it comes to peace negotiations: A first attempt at peace talks failed in 2006 when ETA militants broke a truce and killed two people with a car bomb at Madrid airport.
Formed under the oppressive regime of Gen. Francisco Franco, Euskadi ta Askatasuna (Basque Homeland and Freedom), for four decades has fought for an independent state in northern Spain and southwestern France and has been blamed for around 850 deaths. It is considered a terrorist organization by the European Union and the United States.
ETA's violent resistance dates to the 19th century when religiously conservative Basques disapproved of the too liberal style of governance in Madrid, which aimed for more centralization. The Basque region as early as the Middle Ages enjoyed special privileges and autonomy, although they weren't always fully honored by Madrid.
When the Franco government harshly cut privileges and tried to destroy Basque nationalism, ETA formed itself as a militant resistance group aimed at ending the oppression and installing a fully independent Marxist-Leninist Basque state.