Spanish police say they have practically "decapitated" the leadership of far-left militants Grapo with the arrest of three key suspects.
An international arrest warrant had been issued for Juan Garcia Martin
Juan Garcia Martin, the alleged supreme chief of the terror group, was detained along with two women during a dawn raid in the north-eastern town of Reus.
Grapo is blamed for dozens of deaths in Spain since the 1970s.
Police are investigating possible links between the suspects and the killing of a woman in Zaragoza in February.
Mr Garcia Martin is also suspected of involvement in an attack on a bank branch in Castellon in February, which left an employee injured.
A police spokesman said Grapo, which stands for October First Anti-Fascist Resistance Group, had been virtually dismantled by the security forces in Spain and France between 2000 and 2002.
But they had regrouped to carry out terrorist activities, as was demonstrated by the attacks this year, he said.