Two foreign nationals were still in hospital after being seriously hurt on Tuesday in an explosion that ripped through a hotel in the beach resort of Alicante, followed minutes later by a blast in nearby Benidorm.
Police identified their suspect as 23-year-old Jon Joseba Troitino, who is related to two prominent convicted activists with ETA, the group fighting for an independent Basque homeland in northern Spain.
Spanish authorities fear ETA, which has killed more than 800 people over three decades, is bent on bringing the country's lucrative tourist industry to its knees by waging a summer campaign of violence at popular resorts.
The governor of the Valencia community where the two resorts are located said Tuesday that a man in his early 20s who had reserved two nights at the targeted Hotel Bahia in Alicante was believed to have left a suitcase containing an explosive device in his room before checking out.
Police have distributed a photograph of Troitino and are investigating whether he acted acted alone or had support from an armed ETA unit.
Interior Minister Angel Acebes, who called on
Spanish people to close ranks against the bombers and appealed for information about them, told Spanish radio that Troitino "could have an important link with yesterday's attacks."
"All members of the terrorist organisation ETA who carry out attacks are locked up, they always were -- and this time also they will be."
--Interior Minister Angel Acebes
The suspect is the son of convicted ETA activist Domingo Troitino, alias "Txomin" who was jailed for 794 years in 1989 for a 1997 attack on a Barcelona supermarket which killed 21 people.
He is also the nephew of another incarcerated ETA activist, Antonio Troitino, who was behind a 1986 attack which killed 12 policemen in Madrid.
"All members of the terrorist organisation ETA who carry out attacks are locked up, they always were -- and this time also they will be," Acebes vowed.
A 32-year-old Dutch man injured in the Alicante blast came out of a coma on Wednesday but remained in critical condition, while a 24-year-old German student underwent surgery for neck and head injuries, hospital officials said.
In all, eight people were injured in the Alicante blast while four policemen were hurt in the attack in a Benidorm hotel as they were cordoning off the building after receiving a warning from someone purporting to represent ETA.