The three — a Russian, a Russian of Chechen descent, and a Turk, according to Spanish police — were detained Wednesday. The Turk was arrested in the southern city of La Linea bordering the British colony of Gibraltar, while the other two were picked up near the central city of Ciudad Real as they travelled toward a northern Spanish town near the border with France.
Enough explosive material was found in the house in La Linea where the Turk lived to blow up a bus, and the material could have been especially dangerous if combined with shrapnel, Interior Minister Jorge Fernandez Diaz said.
Investigators found no indications that the three were targeting Gibraltar, he said, declining to offer specifics on possible targets, except that "there are clear indications they could have been planning an attack in Spain and/or another country."
"This is one of the most important operations carried out against al-Qaida," Fernandez Diaz told reporters. He said the operation involved close collaboration with intelligence services from "Spain's allies," without identifying any of the countries.
The arrests came as the Summer Olympics were being held in Britain under tight security against possible terrorist attacks, including military aircraft and ground-to-air missiles.
Spanish authorities had been watching the suspects for "some time," the minister said and decided to arrest them after the two Russians took a bus toward France.
They departed from the southern city of Cadiz en route to the northern town of Irun, possibly planning to cross into France, the minister said. The pair had been in Spain for about two months. Cadiz is near the large U.S. military base in Rota alongside the Mediterranean.
"Police moved to arrest them when it became known that they planned to leave Spain," he said. One resisted arrest and had to be subdued by authorities.
Fernandez Diaz did not disclose the suspects' names, but said the Russians were suspected al-Qaida operatives while the Turk was a facilitator. Pictures of them were released by Spanish authorities, but the suspects were identified only by their initials: C.Y. for the Turk and A.A.A. and M.A. for the other two.
The mug shots showed three men who appeared to be in their 30s, two with crew cuts and one with scraggly hair down to his shoulders.
The minister described one operative as a key member of the terror network and said investigators also found "documentation of the use of ultralights as well as remote controlled planes," without providing more details or saying whether a plot may have been planned using them.
Several of those detained had also "vast experience" with explosives including car bombs plus sniper training and the creation of poisonous substances, said Fernandez Diaz.
Spanish police have arrested dozens of al-Qaida suspects since the Sept. 11, 2001, attacks in the United States and the 2004 train bombings in Madrid.
Most Islamic-based terror arrests in Spain over the last several years have been of lower-level players, although officials in March arrested a suspected al-Qaida member in the eastern city of Valencia described as key to its Internet propaganda and recruiting operation. The Jordanian with Saudi citizen ship was nicknamed "al-Qaida's librarian."
But the detention of the Russians and the Turk was especially significant because of their apparent high level of training, said Magnus Ranstorp, a terror expert at the Swedish National Defence College.
The evidence about ultralight planes was troubling, he said.
"It's always a worry that someone could get a hold of a private plane and try to do a (terror) operation against an event," Ranstorp said.
But getting to Britain during the Olympics probably would have been a challenge for the three, even if they had not been under surveillance.
Passport-free ground travel in much of the 27-nation European Union meant they could have gone to countries on the continent without facing document checks, but getting to Britain is more difficult because passport checks for visitors are mandatory.
Britain "has the tougher border control. They are almost like the United States in some sense. They triple check your name in databases," Ranstorp said.
The three will appear soon before an investigating magistrate at the National Court in Madrid and remain under detention while a judge studies the case and decides on possible charges. That process could take anywhere from days to months, and authorities are not likely to release more details about the case until the judge finishes that work.