MADRID -- The history of terror plots is filled with unusual schemes, from an underwear bomber to model planes filled with explosives – showing how would-be attackers are constantly hatching ideas to catch authorities by surprise. Now questions are being raised about whether two Russians held on terror charges in Spain were planning to launch airborne attacks on paragliders.

The two Russians took paragliding lessons this year in a southern Spanish region renowned for the sport, authorities said over the weekend. A Turkish engineer also under arrest paid for the lessons.

Spain's Interior Ministry declined comment on whether investigators believe Eldar Magomedov and Mohamed Ankari Adamov were using the paragliding lessons to train for an attack. But analysts said the allegation in court paperwork that they took the lessons is almost certainly a key line of investigation.

"It would be surprising if it is not significant in the investigation given the fact that it is mentioned, and secondly because both of them are doing it," said Magnus Ranstorp, a terror expert at the Swedish National Defence College. "Terrorism is a full-time occupation, you don't pursue hobbies on the side. It's such an unusual activity as well that it merits significant consideration."

Paragliding pilots said in interviews that the Russians probably wouldn't have raised red flags at paragliding schools because it's easy to sign up for lessons.

There are no criminal background checks for would-be paragliders; students generally only have to be physically fit and able to speak English or the language of the country where the courses are taught. Europe already has at least 100,000 licensed paragliders, many using gliders that fold up and fit into car trunks.

No attacks have been carried out using paragliders, experts said. The chances of success of such a plot would be severely limited by the difficulty handling the gliders over cities, terror and paragliding experts said.

However, paragliding did feature in one recent stunt by an environmentalist that must have given chills to authorities.

The activist was arrested in May after dropping a billowing smoke bomb onto the roof of a French nuclear reactor. Video footage captured the airborne activist on a motorized paraglider after he dropped the smoke bomb, circling the reactor before making a wobbly descent to the ground with his glider's parachute-like wing emblazoned with "Greenpeace."

Paragliding was banned in and around New Delhi during the 2010 Commonwealth Games "to provide security against any sort of terror attacks," said police spokesman Rajan Bhagat. And authorities in Denmark banned allowing paragliders from flying above Copenhagen in 2009 when the International Olympic Committee met to pick the winner of the 2016 Summer Games.

They prevented the use of the motorized gliders the pilots planned to use because of worries they might be able to land in a small space inside the security perimeter of an opening event attended by Michele Obama and the presidents of Brazil and Russia, said Rasmus Rohlff, general secretary of the European Hangliding and Paragliding Union.

"I asked if we could fly to honor them," Rohlff said. "The Danish police said no, that they were concerned that we would be able to land in a small area, leave something there, and fly off."

At the London Olympics, all aircraft, including hot air balloons and paragliders, need to obtain prior approval from authorities if they want to fly from, into or within the restricted zones of central London and the Olympic park, according to Britain's air traffic control agency, NATS.

The restrictions are monitored by Britain's Ministry of Defense, which can scramble military jets or helicopters to intercept any aircraft that enters the no-fly zone without permission.

Spanish officials haven't said where the Russians took their lessons, but the Turkish engineer was living just outside of Gibraltar in the city of La Linea in Spain's Andalusia region. And Andalusia is a popular paragliding area for tourists from across Europe, said Daniel Blanco of the Andalucia Federation of Air Sports.

"Any tourist can do paragliding and it's an important source of income here," he said. "To learn, they'd just have to speak Spanish or English, and buying a paraglider is easier than buying a car or a motorcycle."

Licenses to fly are granted by paragliding federations in each European country under rules set by civil aviation authorities, with no criminal background checks, said Eugenio de Almeida, president of Portugal's Free-Flight Federation, which covers paragliding.

Learners must provide an ID, an address and a medical certificate, then just a photo to go with the license. They spend about 40 hours in classes and at least 40 hours flying with an instructor to get a basic license.

Taking off with as much as 80 kilograms (176 pounds) of explosives would be possible on a paraglider designed for a pilot and a passenger, de Almeida said. He characterized the idea of terrorist paragliders as "not an unspoken issue" but "not a topic that's on our minds."

"Obviously, paragliding is a mode of transport like any other," de Almeida said.

Attacking a target in a city using a non-motorized paraglider would be virtually impossible unless the city was ringed by mountains or steep hills, Rohlff said. And winds and turbulence inside cities would make it extremely difficult for pilots with motorized paragliders to navigate where they want to go unless they had years of experience. Authorities did not say if the Russian suspects trained on traditional gliders or motorized ones.

"Flying over a city is not the biggest problem, but if you need to land and go down in a city with high buildings it is suicide, but maybe in a terrorist's mind this doesn't matter," he said.

In any event, motorized paragliders are slow-moving, loud and appear on radar covering airspace around urban airports. They are easy for authorities to intercept, usually as a potential hazard to commercial aviation.

"If you enter Madrid, the alarm will go off and the helicopters will go after you," Blanco said.

But experts said the Spain allegation shows that attacks by paragliding terrorists shouldn't be ruled out. The Turkish suspect had also flown remote-controlled model airplanes, Spanish authorities said.

"It sounds like an innovative terrorist tactic to be taken seriously," said terror expert Fernando Reinares of Madrid's Elcano Royal Institute. "Suicide bombers using paragliders may not provoke devastating, catastrophic attacks, but can still cause an important number of fatalities in crowded places and generate widespread panic."

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Associated Press writers Barry Hatton in Lisbon Portugal, Paisley Dodds in London and Muneeza Naqvi in New Delhi contributed to this report.

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