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Three alleged al-Qaeda suspects with possible plans for an attack in Europe have arrived at Spain's National Court for questioning by a judge.
Police with automatic weapons closed streets as the suspects arrived in a police van. The judge was expected to decide Friday after the closed-door hearing if they will remain in detention.
A Russian, a Russian of Chechen descent and a Turk were arrested Wednesday. The Turk was arrested in the southern city of La Linea bordering the British colony of Gibraltar. The Russians were picked up traveling by bus toward the French border.
Authorities say enough explosive material was found in the La Linea house where the Turk lived to blow up a bus. Authorities found evidence suggesting they were experimenting with ultralight planes.
In a news conference on Thursday, Jorge Fernandez Diaz, the interior minister, said: "There is a clear indication that those arrested could have been planning an attack in Spain or in Europe."
Fernandez Diaz said that the operation was conducted with the help of other international police forces, and that the explosive material was being analysed.
Al Jazeera's Barnaby Phillips, reporting from Madrid on Thursday, said that the interior ministry were not offering any more details at this point "but they are satisfied by the arrests and discovery of explosives".
"They believe that one of the suspects arrested is a senior operative in al-Qaeda and is considered a big catch, and there is a lot of satisftication with these arrests.
"It is clear that authorities see this as the biggest breakthrough since the bombings in Madrid in 2004," Phillips said.
The attacks on the capital's train system eight years ago left 191 people dead and 1,800 wounded.
No details were given on how much material was found on Thursday, though the minister said it was enough to blow up a bus.
"The western world faces a global threat from the terrorist organisation al-Qaeda," Fernandez Diaz said.
"Spain does not suffer more or less of a threat than the entire western world. I want this to be clear.
"What is clear is that at the level that it is a global threat, the intelligence services and police in the West, those who together make up our allies, are working very closely together."
Two of the suspects were travelling in a bus towards France when they were arrested in a layby at Valdepenas by a police special operations group, according Spain's El Pais newspaper.
The Turkish man was reportedly arrested in a flat near La Linea de la Concepion where the group were allegedly storing explosives.
They will appear soon before an investigating magistrate at the National Court in Madrid and be detained while the 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.