Foiled terrorist plots in the United States and Spain have New York State law enforcement officials advising people to be wary of anyone with an unusual interest in remote-controlled planes or extreme sports like sky diving, hang gliding and paragliding.

An unclassified report from the New York State Intelligence Center viewed by the Investigative Project on Terrorism pointed to recent arrests in Spain and in Massachusetts in terror plots involving aerial attacks.

Spanish authorities arrested three men last month, seizing "enough explosives … to blow up a bus." A video showing a large remote controlled plane dropping packages led officials to believe the suspects were plotting to attack a shopping center in the British territory of Gibraltar around the time of the Olympic Games in London.

In July, Massachusetts resident Rezwan Ferdaus pleaded guilty to charges connected to a plot to fly an explosive-laden remote-controlled plane into the Pentagon and U.S. Capitol.

"Terrorists and extremists are thinking outside of the box and could use either of these tactics as a method for delivering or detonating explosives, especially at a crowded gathering" like a sporting event or a concert, the New York State advisory said.

A remote-controlled plane can carry 20 lbs. of material while a paraglider theoretically could navigate with more than 175 lbs. of explosives, which would make steering difficult.

Signs of suspicious activity include people seeking lessons in hang gliding or similar sports who withdraw before finishing a course, have sketchy identification papers or who ask unusual questions about drop sites.