One of the busiest airports in the United Kingdom was shut down for 19 hours after drones were seen flying over the runway.

More than 65,000 people at Gatwick Airport had their travel plans disrupted when two drones were spotted around 9 pm on Wednesday night, causing a massive shutdown. Reports say that close to 20 separate police units were dispatched to search the property surrounding Gatwick’s airfield to find the drone operator but have been unsuccessful.

The airport’s chief operating officer, Chris Woodroffe, said that during the closure, no planes were allowed to take off or land. He said more drones were spotted around 12 p.m. on Thursday. Euro Control, the European aviation agency, reported that the airport will stay closed until at least 4 p.m. on Thursday with Woodroffe mentioning that he doesn’t know when the airport would open again.

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Gatwick Airport, in the outskirts of London, is the second largest airport in the UK, with more than 56 airlines operating daily. The shutdown comes at a horrible time as Gatwick expecting close to three million passengers traveling through the airport during the holiday season. This isn’t the first time a drone interrupted the airport’s traffic. Last year, people were first forced to sleep in the airport and grounded planes after a drone was spotted on the runway.

The CEO apologized to passengers, calling the act “irresponsible.” Police say it was a “deliberate act” but there are no indications that the action is terror-related. Woodroffe also said that the police didn’t want to shoot the drones down because of “what might happen with stray bullets.” The airport is warning passengers not to travel to the property before checking the status of their flight. Gatwick is working with airlines to schedule redirected flights and get the message out to passengers.

Flying a drone and disrupting an airport is a crime with a sentence of up to five years in prison.