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 from paragliders.

The two 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 to 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 specialist at the Swedish National Defense College. ‘‘Terrorism is a full-time occupation. You don’t pursue hobbies on the side.’’

Paragliding pilots said in interviews that the Russians probably would not have raised red flags at paragliding schools.

‘Terrorism is a full-time occu-pation. You don’t pursue hobbieson the side.’

There are no criminal background checks; students generally only have to be physically fit and able to speak the language of the country where the courses are taught. Europe has at least 100,000 licensed paragliders.

No paraglider attacks have been carried out, analysts 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, with his glider’s parachute-like wing emblazoned with ‘‘Greenpeace.’’

Spanish officials haven’t said where the Russians took their lessons, but the Turkish engineer was living just outside of Gibraltar in La Linea.

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.

Taking off with as much as 176 pounds of explosives would be possible on a paraglider designed for a pilot and a passenger, de Almeida said.