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