LONDON (REUTERS, AFP) - British police on Sunday (Dec 23) released without charge two people arrested in an inquiry into the illegal use of drones at London's Gatwick Airport that crippled operations for three days last week.

Sussex Police arrested a 47-year-old man and a 54-year-old woman from the local area late on Friday (Dec 21).

"Both people have fully co-operated with our inquiries, and I am satisfied that they are no longer suspects in the drone incidents at Gatwick," Detective Chief Superintendent Jason Tingley said on Sunday.

The two were held after information was passed to the police by a member of the public, Tingley told Sky News. He said he was confident the arrests were justified.

He said authorities were continuing to actively follow lines of investigation to catch those responsible for the most disruptive incursions from unmanned aerial vehicles seen at any major airport.

A damaged drone had been recovered a close to the perimeter of the airport, he said, and it was being forensically examined, for example for clues about whether it was controlled remotely from afar or by somebody in the vicinity.

No group has claimed responsibility for the disruption.

"We have kept an open mind throughout and that is still the case with regards to the motivation behind these incidents," Tingley said.

The airport said on Sunday it was offering a reward of 50,000 pounds (S$87,000) for information leading to the arrest and conviction of those responsible.

The drone disruption at Gatwick Airport affected tens of thousands of passengers during the pre-Christmas getaway.

Drones were first sighted hovering around Britain's second-busiest air hub on Wednesday (Dec 19), grinding the runway to a standstill and causing chaos for more than 120,000 people.

Gatwick, Britain's second-busiest airport, was reopened on Friday after being forced to close for 36 hours.

A statement released on Sussex Police force's website said the probe was ongoing, and officers were using "a range of tactics" to "build resilience to detect and mitigate further incursions from drones".

"We continue to urge the public, passengers and the wider community around Gatwick to be vigilant and support us by contacting us immediately if they believe they have any information that can help us in bringing those responsible to justice," the statement added.

"Every line of inquiry will remain open to us until we are confident that we have mitigated further threats to the safety of passengers."

Police urged passengers and the public to remain vigilant around the airport, south of London, and report any further sightings.

Flights resumed on Friday after a new drone sighting briefly forced planes to be grounded as a "precautionary measure", a Gatwick spokesman said.

Sussex Police said officers had been using "a range of tactics" to hunt for the mystery drone operators and "build resilience to detect and mitigate further incursions" from the device.

The dangers posed by drones include the possibility of a device smashing into a passenger plane or being sucked up into an engine where its highly flammable lithium battery could cause a catastrophe.

Airlines operating at Gatwick, which is located south of London, include easyJet, British Airways and Norwegian.

Widespread disruption

Passengers, many trying to get home for Christmas or to start their holidays, were advised to check the status of their flights before travelling to the airport.

On Friday morning a Gatwick spokesman said 91 of the day's 412 scheduled arrivals had been cancelled, while 64 of 371 scheduled departures had also been scrapped. The army was called in on Thursday to offer support, with the defence ministry deploying what was described only as specialist equipment.

"There are a range of measures which are there today which should give passengers confidence that they are safe to fly," Transport Secretary Chris Grayling told the BBC on Friday. Government officials held an emergency meeting to discuss the situation.

Cat and mouse chase

There had been more than 50 sightings of the device or devices since the first reports at 9:00 pm on Wednesday (5am Thursday Singapore time) and shooting down the drone had been considered as an option, said Jason Tingley of Sussex Police.

"We will do what we can to take that drone out of the sky and remove that disruption," he said. Justin Burtenshaw, head of armed policing for Sussex and Surrey said on Thursday: "Each time we believe we get close to the operator, the drone disappears. When we look to reopen the airfield the drone reappears".

Sussex Police Assistant Chief Constable Steve Barry said officers were working on the theory there was more than one drone. Before Friday's sighting at 5:10 pm, a drone had last been spotted at around 10:00 pm on Thursday.

'We're in limbo'

Some 10,000 passengers were affected on Wednesday night, and a further 110,000 who had been due to either take off or land at the airport on 760 flights Thursday. Mike, from London, had his flight cancelled on Friday and will miss his connection to Ghana.

"We're in limbo. We don't actually know when we'll be flying out at all because we haven't been promised a rescheduled flight, we haven't been promised any further information, any compensation. Nothing at all."

Darcis, 32, who was supposed to arrive from Milan on Thursday and had to sleep at the airport, said: "I cannot understand why such a small thing can cause an international airport like Gatwick (to close). They should be ready for these things. I really don't understand what we can do."

The drama dominated Britain's newspapers on Friday, with speculation that an eco-activist was responsible.

Gatwick, around 50km south of the British capital, is the eighth-busiest airport in Europe and sits behind Mumbai as the world's busiest single-runway air hub. Inbound flights were diverted to other airports, including Paris, while passengers waiting to take off faced gruelling delays.

Under a new British law, drones cannot be flown near aircraft or within a kilometre of an airport, or at an altitude of over 122 metres. Violators face up to five years in prison for endangering an aircraft.

Steep learning curve

Aviation chiefs are going to be on a steep learning curve to counter the security threat posed by drones after what happened at Gatwick.

"This kind of incident is unprecedented anywhere in the world, the disruption of an airport in this way," Britain's Transport Secretary Chris Grayling said Friday.

"We're going to have to learn very quickly from what's happened," Grayling told the BBC. "There certainly isn't a straightforward commercial off-the-shelf solution that automatically solves all problems."

In fact, drone disruption at airports is not quite unprecedented. In 2016, Dubai International Airport closed three times because of drones being used for leisure nearby. The delayed and rerouted flights cost millions of dollars.

In February 2016, an Airbus A320 flying the Barcelona-Paris route reported a near miss with a drone at 1,600 altitude (one mile up) as it approached Charles de Gaulle airport.

Gatwick chief executive Stewart Wingate wants to see swift, coordinated action. "These events obviously highlight a wider strategic challenge for aviation in this country which we need to address together with speed - the aviation industry, Government and all the other relevant authorities," he said.

In 2016, the European Aviation Safety Agency logged 1,400 drone incidents in Europe, up from 606 between 2011 and 2015. Deputies in the European Parliament approved EU-wide regulations on the use of drones, but still need formal approval from European ministers before taking effect.

Until then, it's up to individual member states to set their own rules. In France, civil aviation chiefs have drawn up an interactive map of zones where drones are either restricted or banned. Airports, of course, are off limits.

Charles de Gaulle airport just north of Paris is looking at a long-distance protection system to tackle hostile drones.

But finding high-tech solutions to the drone threat in airport presents particular challenges, says Lucas Le Bell, founder of the start-up Cerbair that specialises in tackling the drone problem. The noise levels at airports, the security requirements and the saturated level of communications make the task much harder.

His team is working on isolating the frequencies used to control drones, so they can not only detect and locate the devices but take control of them.

Elsewhere in France meanwhile, Mont-de Marsan airbase in the southwest has been working on a more low-tech solution. The airforce, inspired by a similar experiment in the Netherlands, has been training golden eagles to search and destroy the intruders.

But that system still needs work: earlier this year, one of the eagles attacked a girl after mistaking her vest for the enemy.