LONDON – London's Gatwick Airport partially reopened Friday after drones caused a security alert that shut down Britain’s second-busiest airport for more than 24 hours. But the chaos isn't over yet.

The airport said in a statement that "a limited number of aircraft" were scheduled for departure and arrival and that these would be "subject to delays and cancellations."

Thousands of passengers have been stranded or delayed at the airport. Many slept on benches and floors and complained of cold and uncomfortable conditions in terminals. About 43 million passengers use Gatwick each year and 110,000 were scheduled to pass through Thursday, one of the busiest days of the holiday season.

The drones were first spotted flying over the airfield south of London on Wednesday.

Britain's Transport Minister Chris Grayling ruled out that the drones were connected to a terror attack and police said there were indications the industrial-grade drones were flown by environmental activists trying to cause chaos for travelers and businesses.

Police said it was a "deliberate act to disrupt the airport."

Authorities were concerned that the drones could collide with aircraft.

Neither the drones nor their operators have yet been located. Britain's military was called in to assist in the search, although police said their recapture, including shooting them down, was a more "difficult and challenging" prospect than it might seem.

"You can't fire weapons haphazardly around an airport," Grayling, the minister, said.

British media reported on how the closure has affected travelers, especially families with young children. "We have now missed our entire trip to Finland to see Santa, the huskies and the Northern Lights," Lauran O'Neill, a nurse who planned to visit Lapland with her daughter, told the BBC. O'Neill told the British broadcaster that her daughter, 5, has been crying non-stop since learning their trip was cancelled.

About 100 flights have already been cancelled Friday. London's Metropolitan Police said they had "significantly increased" their presence at the transportation hub. A normal flight schedule at Gatwick is expected by the weekend, authorities said.

Friday (as well as Thursday) are expected to be the single busiest travel days of the holiday period in the United States, according to projections from both the Airlines for America trade group and the Transportation Security Administration. Passengers are the least likely to endure crowds on Christmas Eve, Christmas Day and Saturday, Jan. 5. Those are projected to be the three slowest days of the holiday flights window.

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Contributing: Ben Mutzabaugh