London’s Gatwick Airport remained shut during the busy holiday period on Thursday while police and airport officials investigate reports that drones were flying in the area of the airfield.

Passengers were advised to check the status of their flights before heading to the airport. The airport said all incoming and outgoing flights were suspended.

Chris Woodroofe, Gatwick’s chief operating officer, told BBC radio that roughly 10,000 people had been affected by the shutdown.

He said drones remained active in the area as late as 0700 GMT (2am EST) on Thursday and that two drones had been seen crossing the perimeter fence and entering the runway area.

Any problem at Gatwick causes a ripple effect throughout Britain and continental Europe, particularly during a holiday period when air traffic control systems are under strain.

The airport first closed Wednesday night at around 9pm (GMT) when drones were sighted. It reopened at around 3am (GMT) on Thursday morning but shut down again in 45 minutes when drones were spotted.

A police helicopter was hovering near the airfield on Thursday morning to monitor the situation.

Passengers complained on Twitter that their flights had landed at London Heathrow, Manchester, Birmingham and other cities.

One traveler whose flight was diverted tweeted that passengers weren’t being told when they could continue to their destination.

Gatwick is a busy airport about 45 kilometers south of London, hosting a variety of short and long-haul flights and serving as a major hub for the budget carrier easyJet.

Gatwick normally operates throughout the night but the number of flights is restricted because of noise limitations. The airport website says it usually handles 18 to 20 flights overnight during the winter months.

Gatwick said in a statement that it apologised for the inconvenience but had to put safety first.

There have been occasional reports of drones nearly hitting commercial airliners in the London area in recent years.

Strong sales of small consumer drones have led to repeated warnings about a possible threat to commercial aviation.