At the time of his arrest in France, Mikel Garikoitz Aspiazu Rubina, 35, was the most wanted leader of the Basque separatist group Eta still at large.
The nickname Txeroki means Cherokee
The suspected Eta military chief, who goes under the name of "Txeroki", was born in the Basque city of Bilbao but Spanish police believe he had lived in France for a number of years.
He is alleged to have planned a series of attacks in Spain and France after taking over Eta's military operations in late 2003.
Mr Aspiazu Rubina is said to be a hardliner behind Eta's decision to return to violence in 2006 after failed talks with the Spanish government. He personifies a radical young generation that has taken control of Eta.
When Mr Aspiazu Rubina was 20 he became involved in organised violence known as "kale borroka" (Basque for street fighting), which Eta uses to recruit radical new members.
Earlier this decade he moved into the notorious "Vizcaya" Eta commandos and was trained by their leader, Soledad Iparragirre Genetxea, known as "Anboto", who was arrested in France in 2004.
Mr Aspiazu Rubina was allegedly involved in two failed operations, a planned car bomb attack weeks before the 2004 elections and a plan to assassinate King Juan Carlos in Majorca in the same year.
Since then police suspect he was connected to all major Eta operations.
Eta had called what is said was a "permanent" ceasefire in 2006.
But Spanish media say that Mr Aspiazu Rubin recruited new Eta members during the ceasefire to try to establish a "new Eta" with young recruits from the kale borroka to revitalise armed operations.
When the secret talks with the Spanish government failed, Eta returned to violence with a bomb attack that killed two people at Madrid's Barajas Airport in December 2006.
It is thought that the Madrid attack was a result of Mr Aspiazu Rubin's increasing influence in the organisation.
A few months later Eta formally announced the end of the ceasefire.
Mr Aspiazu Rubina is linked to the murder of a judge in 2001 and the French interior ministry says he is suspected of involvement in the killing of two Spanish policemen in southern France last December.
The undercover detectives were shot dead as they took part in a surveillance operation against Eta suspects.
Spain's ruling Socialist Party hailed the arrest.
In a statement it said: "This is magnificent news of great importance because it is the chief of the Eta commandos, the person who was behind attacks, who gave the order to kill and who himself killed, a very bloodthirsty terrorist.
"It is very important to have decapitated a very hierarchical organisation like Eta."
Eta is blamed for the deaths of more than 820 people in its 40-year campaign for an independent Basque nation.