LONDON (AP) — Flights resumed at London's Gatwick Airport on Friday after drones sparked about 36 hours of travel chaos including the shutdown of the airfield, leaving tens of thousands of passengers stranded or delayed during the busy holiday season.

The airport said in a statement that Gatwick's runway is "now available and aircraft are arriving and departing.

"We are, however, expecting knock-on delays and cancellations to flights," it said. "If you are due to travel from Gatwick today, we strongly recommend that you check the status of your flight with your airline before departing for the airport."

Airlines also advised customers to check their flight status before going to Gatwick because numerous cancellations and delays are expected. By late morning, some 145 of Gatwick's scheduled 837 Friday flights had been canceled.

The prospect of a deadly collision between what police described as industrial-grade drones and an airliner led authorities to stop all flights in and out of Gatwick, Britain's second-busiest airport by passenger numbers, on Thursday. The drones were first spotted Wednesday evening.

The Gatwick statement suggests authorities are concerned the drones may be seen again, which would likely lead to a fresh closure of the runway. There were repeated sightings Thursday, and authorities decided any flight operations would be unsafe.

Transport Secretary Chris Grayling said there had been about 40 sightings of "a small number of drones" while the airport was shut down. He told the BBC the drone disruption at Gatwick was "unprecedented anywhere in the world."

The last confirmed drone sighting was at 10 p.m. Thursday.

Grayling refused to say what security protection had been added overnight but said the airport is considered safe for flights even though the drone operator has not been apprehended.

Grayling cited additional "military capabilities" and "a range of measures" that had been put in place but would not provide details.

The British military has joined police and aviation authorities in the search for the culprit or culprits behind the drone intrusion, which police said was designed to cause maximum disruption over the holiday period.

Officials say shooting down a drone remains a "tactical option" but there are concerns that any weapon used to attack the drone could inadvertently hurt people on the ground.

The airport, about 30 miles (45 kilometers) south of central London, sees more than 43 million passengers a year. About 110,000 had been scheduled to pass through on Thursday, one of the busiest days of the year.

The runway closure has had a spillover impact on the international air travel system, and thousands of passengers remain short of their destinations.

Many holiday plans were disrupted and travelers stuck at Gatwick described freezing conditions as hundreds slept on benches and floors. Many passengers and their families complained they weren't being kept informed about re-routed flights.

Authorities haven't released details about the drones being used except to characterize them as meeting "industrial specifications."

The motive for the drone use isn't clear. Police say there is no indication it is "terror related."

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Frank Griffiths contributed to this report.