London — Drones spotted over the runway forced the shutdown of England's Gatwick Airport on Thursday during one of the busiest times of the year, stranding or delaying thousands of Christmas-season travelers and setting off a hunt for the operator of the intruding aircraft. Police said they had no doubt the intrusion was a deliberate attempt to disrupt operations at the airport south of London during a peak period but that there were "absolutely no indications to suggest this is terror-related."

Defense Secretary Gavin Williamson said the military would be deployed to help police. He said the armed forces would bring "unique capabilities" but gave no details.

The prospect of a deadly collision between what police called "industrial"-grade drones and an airliner led authorities to stop all flights in and out. About 10,000 travelers have been affected, reported CBS News' Roxana Saberi, including 4,000 who were stuck at airports and 6,000 who were on flights that were diverted. On Thursday, 760 flights were canceled or delayed, including some heading to airports in major American cities including Los Angeles and New York.

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Gatwick — Britain's second-busiest airport by passenger numbers — first closed its runway Wednesday evening after two drones were spotted. It reopened briefly at about 3 a.m. Thursday, but shut down 45 minutes later after further sightings.

Passengers wait around in the South Terminal building at Gatwick Airport after drones flying illegally over the airfield forced the closure of the airport, in Gatwick, Britain, December 20, 2018. PETER NICHOLLS / REUTERS

The airport, about 30 miles south of London, sees more than 43 million passengers a year. Airport officials have been expecting a record-breaking number of passengers over Christmas. About 110,000 had been scheduled to pass through on Thursday.

Travelers described freezing conditions overnight at Gatwick as hundreds slept on benches and floors, and passengers and their families complained they weren't being kept informed about re-routed flights.

"We understand it's an emergency situation, but the lack of information is really surprising," said Vanessa Avila, an American based in Britain who works for the U.S. military. Her mother was on a flight from Florida to Gatwick that ended up landing in the northern English city of Manchester.

"I haven't slept since yesterday morning. We are very tired. It's freezing, we are cold, having to wear all of these coats for extra blankets," said Andri Kyprianou, of Cyprus, whose flight to Kiev was canceled. "There were pregnant women. One of them was sleeping on the floor. There were people with small babies in here overnight. We saw disabled people on chairs. There were young children sleeping on the floor."

Passengers complained on Twitter that their Gatwick-bound flights had been diverted to London's Heathrow Airport, Manchester, Birmingham and other cities.

Luke McComiskie, who landed in Manchester, more than 160 miles from London, said the situation "was just chaos, and they had only two coaches (buses) and taxis charging people 600 pounds ($760) to get to Gatwick."

Meanwhile, the hunt for the perpetrator continues. About 20 police units from two forces tried in vain to find the drone operator as soon as the first unmanned aircraft was spotted above Gatwick on Wednesday evening. Police told airport officials it was too risky to try to shoot down the drones — stray bullets might kill someone.

"Each time we believe we get close to the operator, the drone disappears. When we look to reopen the airfield, the drone reappears," said Sussex Police Superintendent Justin Burtenshaw. He said the newer-generation drones are bigger and have more range, making it harder for police to zero in on the person controlling the device.

Drones could get sucked into a jet engine or crash through a windshield, incapacitating the pilot.

The crisis at Gatwick had a ripple effect on air travel in Britain, continental Europe and beyond as incoming flights were sent to other locations and outgoing ones were stopped.

Police said the drones were of an "industrial specification," an indication they weren't small, inexpensive machines. The larger drones are more dangerous to jets in flight and can stay in the air longer than the models sold to amateur enthusiasts.

Pilots have reported numerous close calls with drones in recent years in Britain, and aviation authorities have warned of the growing risk of a disastrous collision. Britain has toughened its laws on drones, and flying one within 1 kilometer, or 0.6 miles, of an airport is punishable by up to five years in prison.

Gatwick briefly closed its runway last year when a drone was spotted in the area. A drone also briefly led to the shutdown of Dubai's international airport in 2016.