U.S. authorities ramp up counter-drone security for World Cup matches across 11 cities

Fans who hear the whirring sound of a drone over a stadium might see it as a nuisance but law enforcement has long viewed those aircraft as a potential weapon of mass destruction.

Experts say a terrorist could spend a few hundred dollars on a drone, strap some explosives to it and fly it into a target like a stadium full of people.

Authorities have ratcheted up counter-drone measures used at the Super Bowl and other major events, while Congress has given law enforcement broader authority to electronically disable drones or shoot them down.