(CNN) -- Three suspected terrorists arrested last week in what Spanish officials call one of their largest operations against al Qaeda appear to have been interested in targeting a Gibraltar shopping mall.
Spanish security services suspect their plan may have been to attack the British territory on the southernmost tip of Spain from the air. While their planned date of attack is still not clear, any attack on British soil during the Olympic Games would have generated intense global publicity.
A paragliding instructor told police Saturday that Cengiz Yalcin, the alleged cell's Turkish facilitator, requested to be able to take pictures of a Gibraltar shopping mall "at all cost," said Fernando Reinares, a senior international terrorism analyst who was briefed by Spanish security services on the investigation.
Yalcin, who worked as an engineer at a contruction company on Gibraltar, was arrested in La Línea de la Concepción, a town bordering Gibraltar. He had lived in Spain with his Moroccan wife for several years. Explosives were found in his residence, as well as videos and photos suggesting the possibility of attack preparations, Reinares said.
Spanish security suspect the cell was testing a remote-controlled plane as a potential bomber. Spanish investigators found a video in which Yalcin was flying a remote-controlled airplane, according to Reinares.
The footage showed the plane, which was about three meters long. being maneuvered into a descent. Two packets were then seen dropping from either wing of the plane following his command.
"In the images he can't help expressing his joy for the successful try," Reinares told CNN. "Terrorist innovate and adapt to security measures, we have to always keep this in mind," he added.
It is not the first time that terrorists have sought to use remote controlled aircraft in terrorist plots. In July Rezwan Ferdaus, a Massachusetts resident inspired by al Qaeda's ideology, pleaded guilty to a September 2011 plot to fly a remote-controlled plane into the Pentagon and U.S. capitol with high explosive.
Two Chechen-Russians -- Eldar Magomedov (also known as Ahmad Avar) and Muhammad Adamov -- whom Spanish security services suspect had been tasked with carrying out an attack, were arrested on a bus travelling towards France.
Investigating Judge Judge Pablo Ruz ruled Sunday there was sufficient evidence to unconditionally detain both men. They were charged with membership in a terrorist group and possession or storage of explosives. Yalcin stands accused of possession of explosives substances.
According to a statement released Sunday from Ruz, the Chechens hid their true identity after they were arrested, but Spanish authorities were able to establish their real names after help from Russian authorities. The judge noted that U.S. authorities and Gibraltar law enforcement also assisted the Spanish investigation.
The arrests were announced Thursday. Experts say the men appear to have constituted one of the most skilled and experienced terror cells seen in recent times, and appear to have been dispatched by al Qaeda to carry out an ambitious attack in Europe.
Magomedov, the suspected leader, was a former member of Spetsnaz, the Russian special forces, according to Spain's Interior Ministry. He had training as a sniper and was an expert in poisons, the ministry said.
Reinares, of Madrid's Elcano Royal Institute, said that according to information passed to Spain by several Western intelligence agencies, Megomedov joined training camps in Afghanistan and Pakistan, including camps run by Pakistani militant group Lashkar-e-Tayyiba, after leaving the Russian special forces outfits. According to this intelligence, between 2008 and 2011 Megomedov operated in the southwestern Russian republic of Dagestan and the Pakistani tribal districts of North and South Waziristan, transiting between them, Reinares told CNN.
Adamov, the other Chechen, had received explosives training in Afghanistan, where he become an expert in managing explosives and may have participated in a recent bomb attack in Moscow, according to Spanish authorities.
According to Reinares, the French described the Chechens to their Spanish counterparts as "really dangerous." He said British intelligence services were also involved in tracking the suspected terrorist cell.
Also found in Yalcin's home was equipment for three motorized paragliding machines. Yalcin told a Spanish investigating judge Friday that he was an enthusiast of motorized paragliding and wished to teach his two Chechen associates how to fly them.
Reinares said Spanish security services have established that both Chechens received motor-paragliding lessons near La Linea and may have had some instruction before arriving in Spain. A paragliding hand book in Russian was found in their possession, Reinares said.
Yalcin, the Turkish suspect, was ordered detained immediately after his arrest. Because information obtained by intelligence agencies is generally not admissable in Spanish courts, Spanish police and security services had scramble in the past few days to provide sufficient evidence to bring a case, according to Reinares.
French security services tipped off their Spanish counterparts about the probable arrival of the two Chechen suspected terrorists in May, according to Reinares. The French had been tracking the duo and monitoring their phone calls. At one point, they intercepted a phone call in which the Chechens described Spain as a "more easy country to get explosives," Reinares said.
Investigators moved to arrest all three men after the two Chechens appeared to be heading back to France by bus, concerned that France may be their target. One of them violently resisted arrest, the Spanish Interior Ministry reported.
But police were frustrated by a judge's refusal to grant an immediate search warrant for Yalcin's apartment after his arrest, according to Reinares. Investigators feared that may have given the cell time to dispose of additional explosives, he said.
It took eight to nine hours for the search warrant to be granted. Police then drove with Yalcin and an official of the Spanish judiciary to his residence. According to the subsequent judicial report, his wife greeted him by saying "do not worry, honey, I cleaned it all." The agents wrote they noticed a "very strong smell of bleach when moving into the place," the report said.
Reinares said French security services had asked their Spanish counterparts not to share intelligence on the suspected cell with the Spanish judiciary, fearing that open exposure of such information in court -- as required by Spanish law -- could blow the whole operation. Dog teams used in the search suggested that more explosives had been present than police found, according to Reinares.
Spanish security services believe another target of the alleged terrorist cell may have been a joint U.S.-Spanish naval base in Rota, Spain, near where they were based, and have not ruled out other possible targets in Europe.
"The case definetely stresses, once again, how critical cooperation between Western nations is to successfully prevent and combat international terrorism," Reinares told CNN.
Journalist Javier Ruiz Perez contributed to this report for CNN from Madrid.