Flights in and out of Britain's second-busiest airport are suspended because drones were spotted flying in the area. Ten thousand passengers are affected by the delays and cancellations at Gatwick Airport, south of London, including 4,000 who are stuck at airports unable to depart or arrive, and 6,000 who were on flights that were diverted.

Airport officials said the shutdown started around 9 p.m. Wednesday after two drones were spotted flying over the airfield.

The runway re-opened about six hours later but closed again after 45 minutes following another sighting of drones. The airport's chief operating officer called it a "deliberate act."

Twenty police units have been searching for the drone operators, but officials can't shoot down the drone because of the danger of stray bullets.

Airport officials have been expecting a record-breaking number of passengers over Christmas.

But on Thursday alone, 760 flights through Gatwick were being delayed or canceled. Some of them are heading to big American airports in cities such as Los Angeles and New York.

Flying a drone within around three-fifths of a mile of an airport in the U.K. is illegal and punishable by up to five years in prison. Police have said they don't believe this is case of terrorism at the moment, but we do know that terror groups have threatened to use drones as weapons. This is something aviation and security officials have long feared happening at airports around the world.