Police are questioning a man arrested in Belfast city centre as part of a probe into Basque separatist activity.
The 40-year-old suspect was detained yesterday evening under terrorism legislation in a joint operation between Spanish police and officers from the PSNI's Serious Crime Branch.
Spain's interior ministry has said that the man arrested is Fermin Vila Michelana.
The ministry say he is on an EU wanted list and part of the 'Madrid command' in 2001 that was linked to four car bomb attacks.
The Spanish interior ministry has also said he had links to ETA for a long time before, and had first been arrested in 1989.
Spanish police are expected to travel to Northern Ireland to question the detained man.
In March, a Belfast judge ordered the extradition to Spain of a former ETA leader to face a charge of justifying terrorism.
Jose Ignacio De Juana Chaos, 54, lodged an appeal and was out on bail before a hearing due in June.
However, a court hearing at the end of April was told he had disappeared after not reporting to police for more than a month.
His appeal is due to be heard in his absence on Monday.